[_carbyau_]

Mick Doohan, world champion multiple times broke his ankle while racing. They figured out a thumb operated rear brake for him.

Now, while not exactly common in use, it is a commonly accepted option in racing. Current moto2 championship leader has a rear brake lever on the handlebar.

I imagine at the motogp level there might be biomechanical advantages to a hand lever but I'm no racer so I have never really investigated.

"motorcycle thumb brake kit" gets a few hits to buy your own to fit to your bike if you wish.

[raihansaputra]

Just off the top of my head, but having the option to shift your bodyweight using your legs is quite a big advantage. At MotoGP leved it's either they can exchange that advantage with something else or it's so advantageous everybody will use that.