Such a thing is there to make sure that Mahdis don't keep popping up.
LOL as you know every so often a person gathers a few followers and makes such a claim even now. Knowing full well the conditions.
Imagine what would happen if that was not the condition..we would have mahdis fighting eachother all over the globe claiming the caliphate.
which one of the claimants of being Imam Mahdi AS resembles the one mentioned in Ahadith? Do you reject the lunar and solar eclipse prophecy for the coming of Imam Mahdi AS? Him NOT CLAIMING will be a huge issue for you people.. you people will KEEP ON switching imam mahdi , one after another, thinking each one of them to be theee one. Claiming of being Imam Mahdi and seeing the heavenly signs is enough for a God fearing person to accept the Imam that prophet Muhammad PBUH gave us the good news about.