Because everything is tied into your browser. When you uninstall, you throw everything away if you didn’t have a backup or have it synced to other devices.
Brave Wallet is not Brave Rewards. So unless you sent BAT to your Brave Wallet or you purchased it within Brave Wallet, it wouldn’t be there. If you were Unverified in your Rewards, then your BAT was kept as vBAT in your browser. When you uninstalled Brave, you would have thrown away any vBAT you had.
There never has been. Brave doesn’t have accounts like Google does. Or are you referring to something else?
If it wasn’t linked to Uphold or Gemini, correct. Only exception is if you’re on Desktop and you have an old backup of your \brave-browser folder, at which point it would have the vBAT remaining from then. But I’m guessing it’s unlikely you have that.
Not sure you’ll have to worry about this going forward. Next month, you no longer will be able to earn BAT for yourself unless you are connected to a custodial partner. vBAT is going away. You can read more on that at https://brave.com/rewards-changes/