If people can learn the Snapchat UX, they can learn some SMS text adventure style Uber.

People can be illiterate, dyslexic, inebriated, dumb or with poor command of the language. Good luck with free-text interface.

You'll pretty much would have had human operators over every interaction. That very much hinders the Uber-like growth of the service.

If you can support 95% of the market (with mechanical means), you can capture 95% of the market.

That wouldn't be 95% of market, more like 50-60%, and a strain to use even for those apt. Not everyone is an Infocom playing nerd.

Normal people use SMS just fine all the time.

Falling back to a human operator isn't that bad either: just charge people a little bit extra to talk to a human.