Because of plugins. Browsers have more plugins than most other software and the bitness needs to match between all of them. And saying the flash has been dead

Because of plugins. Browsers have more plugins than most other software and the bitness needs to match between all of them. And saying the flash has been dead for the greater part of a decade is just completely wrong. You have to be just really, completely out of the loop if you think that applies to any kind of reality for general users.