Well, they do own all the Pokemarts now. Since there's nowhere else to shop, it's basically a hospital mixed with Walmart.

Warning! Headcanon incoming!

Aight, so you know how Nurse Joy is always catching flak for saying "We hope to see you again!" when you leave? Well, she's got a reason for saying that.

See, since Pokémon are such an indispensible part of life in that 'verse, the Pokémon Centers are funded by the government. (What's the government like? Dunno, haven't thought that far ahead.) The more patients they treat, the more funding they get. Nurse Joy wants to see you again because you mean more funding for her workplace. This also (almost) explains the shack-like Center that FMA 155 posted. It's on Victory Road. Not many people travel there because it requires an elite amount of skill to even make it there, much less compete there, so they don't get much business, and as a result, don't look as established.