Do keep in mind Dino-Riders was a toy release from 1987 to 1990 and since they used many hardened rubber parts this may also be why they get sticky over time.
Sun exposure can also worsen such cases and moisture levels in your living room etc.
I also have my DR collection displayed in my hallway where no sunlight can breach them.
Other than that dunno why this thread was posted here in the Dino Riders section hah, must of been an accident hehe