Its 10 signiatures iirc, and the reason is because it is an out of date system from the EQ2 era of MMO's. Blizzard never found it necessary to change it because it was commonplace to have one solidified leader as guildmaster who oversaw the day to day operations of the guild and no one questioned it. tey may have thought of making some sort of oligarchy system but probably decided against messing with the coding, and also because it could potentially create problems with coups overthrowing one or more of the leaders in the guild, creating unending shitfests of complaints.
All in theory, but its more than likely coding and laziness to inovate a tried and true system, however easily abused it always is.