It's been proposed. It's a stupid idea for many obvious reasons, but it's also easily defeated by exchanging all my money before it's going to expire with others whose money is also going to expire. It just happens that we did similar amounts of work for one another around that time which we feel deserve similar amounts of repayment. Clock reset. It also doesn't do a thing to prevent criminals from using it; criminals want to spend their gains too, so how does preventing saving do anything against them? Of course, you could build in lots of extra features to make it harder for criminals to use it, most of which would be detrimental to and detested by law-abiding users.
A lot of effort has gone into encouraging people to save money because people who have money available when they need it tend to do better than people who spent all of it. Forcing people who want to save to spend it anyway kind of breaks all of that, generating exactly the opposite of the benefits from it. You'd also get some fun bonus problems from people who didn't need to be convinced to save and now face a use or lose situation. You also get two groups of angry people who might want to attack you because either you took away their money, or to avoid you taking away their money, they invested it in something that doesn't expire but wasn't a good choice. True, that second one normally isn't your fault, but since they had to invest or you would have confiscated it through automatic expiration, they'll ascribe any downstream negative consequences to you. Meanwhile, I see no positives whatsoever.