Star Wars Weekend was a limited time event. Having Rey as a permanent presence would probably entail a lot more than finding a couple of girls who look enough like her and can carry off the performance aspect. Star Wars characters with a more permanent presence in the parks are generally either masked or wear heavy makeup (or are droids). You don't ask Darth Vader or Kylo Ren to take off their masks, and if you do they can refuse while staying in character. If Rey wore a mask to camouflage a performer who doesn't actually resemble Ridley, how often might she be asked to remove it by kids or other guests?
Captain America and Thor began their existence as comic book characters. Although they do appear to hire guys who somewhat resemble the Chrises, maybe a little more slack goes to them because of their comic book backgrounds. Rey would have to be perfect.