I guess there is an argument for giving guys time to prepare properly. Bout agreements need two names on the paper. But with the way things are going with fighters dropping out constantly you could say that fighters prepare more for the date on which they are ready to fight than the opponent these days.
IMO so long as both fighters find out at the same time who they are fighting, then they both have equal time to prepare for that specific opponent. If a fighter isn't ready to take a fight on short notice, they should just say no.