Because that's the reason of a "guild" ... maybe i am wrong but i think the cap should remain.
No one is saying guild size should be infinite, just that we should be able to increase the size. Caps can remain, we just think there should be more flexibility. They could balance it around a guild bank system, and have slot expansions cost a sum of pokedollars or something so only active/contributing guilds that actually need the feature use it, and only a limited number of expansions allowed. Our guild is packed with people spread out over several time zones but we are a pretty happy family and our members want to invite more friends but we are full up usually.