Themes with built in page builder

Hey guys I have been seeing a lot of premium themes like avada for instance, they don’t come with a page builder integrated but install a seperate builder plugin for that, now why is that?
I have used lot of these premium themes on client projects and these visual builders are mostly filled with bloatwares and really put a toll on the performance. The backend edits takes so long with visual composer or other drag and drop builders. Wouldn’t it be better if themes came with page builders integrated rather than as a plugin?

I have been using couple of themes on client projects
divi – https://www.elegantthemes.com/gallery/divi/
and
tl-corporate – https://www.themelines.com/product/tl-corporate-responsive-wordpress-theme/

now these themes come with page builders integrated rather than as a seperate plugin and i must say the performance is way better than themes that use plugin type page builders.

What i feel is that they are free of bloat wares and are shipped with features only required for a certain theme so no unnecessary js or css or functions. Now that’s just my opinion, any thoughts on why the theme industry is like that? is there any good reason behind it?

Any suggestion would help me choose a better theme next time for my client projects.