I'm not sure about that. Targets funds fees are generally a percent not flat, you have less control over your money (which if you're doing a robo-advisor you probably have somewhat bespoke investing requirements) and they only make sense for tax-advantaged accounts (see [1]).

Plus, people invest for reasons other than dated retirement targets.

[1]: https://www.wsj.com/articles/vanguard-target-retirement-tax-...