Because that's the Creeper's personality. It's not explained further than that.
We find that Creeper needs to replenish body parts in order to continue on with his "return every 23 years" cycle. But we do know events that indicate his "feelings": (1) Creeper is particular, and (2) he likes his trophies. That is probably why he saves and displays them.
- He is particular about who exactly will "donate" what parts of their body to him. We know this because Trish volunteers herself to save her brother, but Creeper does not accept. We also see him "smelling fear" in his victims that somehow tell him who is the best match. (This may be from the sequels.)
- We can be sure that the victims do not go willingly. Some are more successful at resisting than others -- making Creeper work harder to earn his body part. It's pretty apparent that Darry resisted quite a bit. At the end of the movie, we see Creeper playing his signature Jeepers Creepers song ("Where'd you get those eyes" -- see YouTube clip at bottom) and then he stands behind his disemboweled trophy so that we can "see" where his eyes went. Darry's remains have not been discarded; Darry is held in an upright position, indicating what his fate will be. (See clip -- graphic)
Caution: Very Graphic Clip
The Jeepers Creepers song