Facebook and Twitter want everything done in-app. so they include the whole kitchen sink, including their own bespoke browser (although Twitter t.co links open in Safari sans extensions). Facebook wants to know every link you click on in their app and attach a bunch of tracking params to the URL so the destination site knows who 'you' are.
Facebook is also very feature heavy and they don't apologize for that. Every week something new is added (like in their recent 'favorites' feature and short video Reels crap). They need to be bloated because they are bloated apps by design. It would be nice to have 'lite' versions of the apps however.