I believe that it is hard for departments and search committees to believe that someone is really willing to start at square one, frankly. And it might seem easier than it really would be, post tenure. People are not always good at forecasting how they will feel about things in the future. Keeping in mind that members of search committees are themselves often tenured professors they might have a very hard time imagining that someone is really willing to give up tenure (and ANY salary $) to make a move, particularly if they view that move as horizontal rather than very, very vertical. Imagine if some of those individuals had a hard time in the tenure decision. They cannot imagine someone would really go through that again. Before a person has tenure it might seem like an easy thing to throw away. After you have it, loss aversion rears its head, believe me. After a few years with tenure, a faculty member has hardly been evaluated at all for a long time! Getting back into the mindset of being evaluated like that becomes (potentially) nauseating and insulting and demeaning. :) So, I think it is possible faculty would think that a person who expresses such a willingness is either foolish or kidding themselves: When it comes down to the final decision that person will stay put. Further, at that decision point, a school that considers recruiting someone who has tenure for a nontenured (but tenure track) position will always be at a negotiating disadvantage relative to the home school. (The candidate would be as well: He or she cannot come back to their home department and say, "I have an offer from X" and hope that the home institution will pony up much, after all they have already given the person tenure….). So, back home the person has more money, more stability, and has already garnered the strongest indicator of the high esteem of his or her colleagues (tenure). The new school can offer none of these things.
As someone who has served on numerous search committees (R1) I will say that applications for assistant level positions from newish associate professors are not common but do they occur. And some of these individuals do mention willingness to give up tenure. But, honestly, sometimes their vitas reveal the reason they want to move (i.e., their research program has suffered in an environment that is not particularly conducive to research). For the very same reason, their vitas do not compare particularly favorably with new PhDs and fresh post docs.
I have a friend who has served as chair in a SLAC psych department. My sense is that in such schools there is a great deal more of a sense of community among the institutions and, perhaps, more concern about snatching another department's star scholar. For what its worth, I think that at R1 schools the notion that faculty shop themselves around continuously is a given; For SLACs, doing that is viewed, potentially, as disloyal or somehow just not polite. I may be mistaken on this, but that's the impression I get.
Finally, I would note that sometimes the 5th year applicant can be viewed as simply setting up his or her back-up plans in case tenure falls through. So, I think it is important not so much to sell the idea that you are confident that you are going to get tenure where you are (because anyone can say that) but to have very specific reasons the new place is appealing to you and provides something you couldn't have where you already are, regardless of the timing. Does that make sense?