That’s the question I’m often asked about BuddyPress, maybe you wondered about it too 😁. We’re not sure there’s a specific reason explaining this fact. As it was necessary before we introduced the BP Theme Compat API (see the 1.7 version announcement post) to use a BuddyPress compatible theme like the one we bundle by default (BP Default), I think we kept the way this theme was loading these assets into the first Template Pack (BP Legacy) we added to BuddyPress.
Let’s only load what we need when we need it… progressively!
add_filter( 'bp_enqueue_assets_in_bp_pages_only', '__return_true' );
If using the above filter, you notice something is going wrong with your website due to the use of a specific BP plugin or theme, report it here and we’ll then have another development cycle to fix things before we completely restrict these assets to BuddyPress generated pages in a second step & in version 12.0.0.
To read more about the story of this change, you can have a look at this ticket #8679.