There is no good reason for copyright to ever expire. That is to say, the economic framework that's well-accepted in policy circles has no way to express the value of letting copyright expire. So copyright keeps getting extended, because it's "obvious."

Let me put it another way. Why don't physical property rights expire? The argument of: "well then other people could use them!" doesn't fly, because the same thing applies to physical property. So long as Disney keeps making use of Mickey Mouse, what economic reason is there to give someone else a crack at the character? Would it create more economic value? What economic theory tells us that?

You can talk about "bribery" all you want, but at base there is a theoretical problem. The accepted belief is that there are no downsides to protecting property rights, and there is no way to express the value of allowing property rights to expire without sounding like a crunchy hippie.