Why? They get compensated when fired anyway
Keep their reputation, get a move to a bigger club, don't get a reputation as unable to work at a higher level.
Two fine examples, Warnock and McCarthy. Both taken clubs to promotion when others hadn't, punched above their weight - the inevitable happens and the clubs are relegated, struggle, and the call goes out that they're incapable of managing at that level, so they get typecast as getting jobs at fading clubs who want to pull their budgets in.
Better,last season, for Robins to have heroically missed out on promotion, narrowly, and then have a half-decent start this season, get a job at a better funded club in this division, go on to be successful.