The candidate has just as much if not more ambiguity and uncertainty as the employer, so why is the burden on the candidate?

It's very possible for a candidate to be thinking something like "I'm currently making $200K. I think it's possible I could get $300K, but $250K would probably be acceptable, and I might even settle for a lateral move of $200K if I find the position really appeals to me." What should they say? $300K and possibly price themselves out? $200K and possibly lowball themselves?