1. These browsers can barely add their own functionality on top of upstream, and maintaining Manifest v2 compatibility may be expensive. Consider that the development of Chrome exceeds 1 billion per year.
2. They all use Chrome's Webstore as a distributing channel, except for Edge, but IMO, Microsoft has an even bigger interest in seeing uBO die.
Brave itself has ad-blocking built-in, which won't be affected, and it's fairly capable, but promising that they'll keep compatibility with uBO is a lie, if they ever made that promise.