Why isn't 6 face buttons a standard?

[Wildcard36qs]

ktay95 said:

Eh, 6 buttons is much better for fighters which btw you dont use your thumb for all 6 buttons but use 3 fingers like playing with an arcade stick.
Otherwise the real issue is that more controllers arent made like this

We use our thumbs for alot as is on a controller but dont use 4-6 fingers at all which are perfect for these extra paddles

I've always wondered, can those rear buttons be additional buttons? Or do they have to be mapped to existing ones?