Aman answered it flawlessly. The reason is because of government regulations. For Brave to allow Brave Rewards on Brave Wallet, they would need to KYC/AML all users…which means in order to use Brave Rewards you’d have to provide all your driver’s license, full address, social security number (or equivalent depending on location), and so much more. It would be crazy to force that just to use a browser.
On top of that, Brave would also be required to obtain certain types of licenses and all, such as money-transmitter license. You can see where Brave addressed this at https://github.com/brave/brave-browser/wiki/Wallet-support-FAQ#why-cant-my-wallet-receive-my-rewards-how-about-tipping-from-my-wallet