Probably because chromium based browsers are often marketed as completely separate ones. Browsers based on it want to make the impression that they are a unique browser with outstanding functionality etc.
Edge? It’s Edge, OUR BROWSER, YOU DON’T NEED ANYTHING ELSE.
Vivaldi? It’s Vivaldi, “we built the best browser with most features blabla”.
Opera? It’s Opera, “better, faster and more secure than a default browser”.
In contrast, librewolf.net’s first sentence is literally “A custom version of Firefox…”. Mullvad also says it’s a collaboration with Tor, and a paragraph down they explicitly say it’s Tor, and therefore FF, based.
Many users, me included, don’t see Librewolf or Tor as separate browsers, because the former is basically just a preconfigured FF, and the latter adds an access point to the Tor network and removes some privacy/security harming features (eg. Sync). The Browser, its features, design and features, stay the same.