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.