Because after a while is supposed that the investment paid off. Why should a single company keep Mickey or any other IP right hostage forever?
I realize that's not what you meant, but acting as though Mickey Mouse becoming public domain will "take away" from us, in the same way giving away the house you live in will "take away" your home is at best a total fallacy.
IP is not the same as physical property. Disney isn't forced to give away they artwork they've made internally. Mickey Mouse as IP, i.e. as a concept, is currently not allowed to be reproduced.
If we're going to debate property rights, it's important to have the context and the property properly defined. Many people can rightfully ask if land should be allowed to be held by any single entity for infinite time, but again, that's not the question here.
In China land use rights can only be bought for a limited time - after this time it goes back to the government:
> https://en.wikipedia.org/w/index.php?title=Chinese_property_...
In this sense other countries than the US indeed give quite different answers to your rhetorical question.