When I'm looking to the keyboard of my phone or tablet, I also don't need feedback, however while writing on a desktop PC or workstation and writing code full speed, physical keyboard gives my brain hints for alignment and takes another concentration task from my consciousness.

Another effect is the screen size. You can see the keyboard and text area in your vision on a phone or tablet, but it's not possible in a desktop or laptop.

Looks like it boils down to the information that we can gather about our fingers and hitting targets with the vision and other senses during writing.

Funny thing is, I can write faster while I'm listening to music and cannot hear the keys' sounds.