Speed. They usually launch fast and are responsive, because there are no graphical resources that need to load or complex widget hierarchies that need to be constructed. There's no need to switch windows, because the TUI launches in the same window you typed the command into. You don't have to take your fingers off the keyboard because having a mouse is not expected in a TUI.

In principle, all of these things could be done with a GUI, but in practice, they're not, and with the phoneification of software, the keyboard is an afterthought, not the main input mechanism.