Well, that’s all well and good IF you happen to play keyboard. Not every harpist can play piano too.
That is one of the reasons they make different kinds of controllers. If you play piano you can use a midi keyboard controller. If you play guitar you can use a guitar equipped with a Rolland system. If you play a wind instrument you can use a wind controller.
You don’t have to relearn how to play in order to use MIDI.
I can mess around on piano a bit, but I can’t play piano anywhere near as well as harp. If I am going to play using MIDI why would I want to have to spend the time and effort to learn to play keyboard better when I can already play what I need on a harp controller?