Yes, in a perfect world with unicorns and rainbows, the faculty would be able to review tenure and revoke it with no fear of retribution.
In reality, it will never happen as in many places, most of the voting majority of the unions are...wait for it...tenured faculty. Asking the majority to vote to amend the contract such that it provides a means of them being fired? Not gonna happen.
Also, even if this contract amendment actually could pass a union vote, guess who would be sitting on the faculty tenure committee? Yep...you guessed it, tenured faculty. Keeping the membership of that committee anonymous would be impossible in small departments. As it stands now, most if not all faculty tenure committees can only recommend tenure, the principal or dean is usually the one that makes the ultimate decision and they can ignore the committee recommendation.
So if you were to try to oust a bad egg, but they happen to be on the good side of the dean, they won't get booted. After that, YOUR ass is going to be the next up for tenure review in very short order. And guess how that's going to go?
This is called politics, and it sucks.
There is a lot of conflict in this situation. In a perfect world, there would be no tenure system, but the pay and benefits would be high enough to attract professional talent from their field. Such a position would be no more or less safe than an industry job, and would attract the people who really want to teach. And they would keep their job based on merit.
But the administration doesn't like this idea, because their payroll will have to explode in size. The dollars win. So short of a faculty member raping and shooting their students, there is a lot of pressure to maintain the status quo, on both sides of the union contract.