No they shouldn't, because such laws will just open the door to even more abuse. Imagine you are raped by someone. You go to the police to report the crime, but unfortunately, they deny, and you can't provide any real evidence. They can now countersue because you've made a "false allegation" which ruined their life.

Presumably for a "false accusation" conviction one would have to prove that (1) the accusations are false, and (2) the accuser was aware that the accusations were false. Merely failing to prove accusations (such as when there is insufficient evidence) would be insufficient for a false accusation conviction, as neither of these requirements are satisfied.

Isn't this already covered under libel?

You're not talked about a false allegation, you're talking about an unproved allegation. Those are very different things.

Falsifying a crime and not having enough evidence are very different things. It takes a lot of work or luck to prove that intent.