Because what is called 'existence' is an almost incomprehensible chaos of interaction... where everything appears to be linked in some way to everything else... endeavouring to define metaphysically what might be understood as an 'entity' or a 'thing' or a 'one' will probably always be frustrated by the complexities of the attempted simplification. Deciding where the space-time boundaries of a seemingly independent thing are, is never going to be defined absolutely by means of a linguistic statement. Much communication and interaction takes place of course without ever having to reflect upon or resolve such an issue, but never-the-less, some basic and 'down-to-earth' assumptions and axioms might help to deal with communications about those unique aspects that we call 'things' or 'entities'.

An entity has a life-cycle. No 'thing' that we have ever become aware of is able to exist forever. Everything we have encountered has a beginning, a space-time life-path, and then an end. Claims for immortality and eternal existence are either unsubstantiated or nonsense. An entity is part of the cosmos and complexifies by interaction with the cosmos. It's form will change with time, but it endures as a material or perceptual continuity. It has an intrinsic formal aspect which the mind becomes aware of in its memory, or can reconstruct from recorded archives. An entity is a transient integrated material continuity that has a beginning, a unique spatio-temporal pathway, and an end.

The form of an entity may change significantly during its life-cycle, and yet we are obliged to assume that something endures in time and space that is the 'same'. The form of a particular biennial Lunaria annua continuously mutates from dormant seed thru germination, growth, flowering, fertilization, the production of seed and then the final disintegration of death. Its 'oneness' as an entity is conserved by the material continuity of its cellular structures whilst it mutates thru the successive phases of its life-cycle. The multiple sequential images of the plant are recorded images of the same entity... not different entities... and serve to archive the life-path existence of that particular entity. Generally, the most pragmatic and uncomplicated perspective would be to consider that... for the most part... only existing material things are entities. Other possibilities however, are difficult to exclude. Is there a metaphysical purpose to considering a musical note to be an entity? A musical note begins when a fluid is made to transmit a vibration, is sustained in the memory of the listener and ends when the oscillations cease. It has a beginning and an end, a continuity in the senses or in a recorded medium, a life-cycle and an existence. It may very well even have a name like 'A 440'. There would be a plausible argument to call it an entity if the motivation was there. Be that as it may however, it remains the case that we are obliged to consider that entities have a continuity of sameness in order that the 'names' we give them and our simplifications of the cosmos are to some degree comprehensible.

Modifying entities disrupts their life-path. Entities interact with and are continuously modified by their environment. At the extreme end of the spectrum, entities are entirely destroyed. Cutting a seed capsule in half destroys the capsule. Separating the hammerhead from the handle de-entifies the hammer. Predators eat prey... comets wipe out dinosaurs... guided missiles destroy buildings... guillotines cut off heads... and so on.
Less violently however, new entities can result from the 'restructuring' of old entities. Replacing the broken handle of a hammer with a new one for example, creates a new hammer entity from an old head entity and a new handle entity. To suppose that a hammer is the same entity, even after it has been fitted with a new handle, serves little purpose. A plant that can be propagated by division, will cease to be that plant when it is divided, and be the source of the beginning of one or more new entities. Partitioning a country destroys the original country, but creates different entities in its place. The end of some entities is often the means whereby new entities have a beginning.
Entities can be integrated into a system and participate in the creation of a new and distinct entity. Component entities are assembled into new devices... like cars, television sets, bridges and houses... Individual insect entities participate in the construction of their nest entity. Element molecules combine chemically to create new substances. One way or another, 'new' entities are always recombinations of 'old' entities.
Minor changes to some entities often make little significant difference to the course of events. Painting the handle of the hammer will not unduly effect its capacity to hit objects. Picking one of the blooms of the lunaria will probably not effect its ability to produce seeds from the remaining flowers. Shearing a sheep doesn't usually mean the end of the sheep. The decisions that humans make about whether a particular entity continues to be the same entity or has reached its 'end' and mutated into a 'new' entity, is usually one of entirely pragmatic convenience or metaphysical inclination.

The existence of some entities can only be inferred from credible scientific evidence. Whilst not directly perceivable, the probable existence of 'atomic' entities may be inferred by observable effects presumed to be causally related... electrons, neutrons, photons, and so on... The existence of a planet around a star in a distant galaxy is sometimes assumed from the periodic variation data of the magnitude of the star. The probable existence of a Coelacanth fish was inferred from seventy million year old fossils. 'Father Xmas', of course, has never been observed directly by any sober and reputable investigator, but during the Xmas calender period, an exponential increase in the piles of decorated wrapping paper, and the significant deposits of airborne reindeer faeces, are considered by most observers to be incontrovertible evidence for his existence.