For the short term, that's certainly true. Long term not so much, because one deviates more and more from public sources. Eventually it's no longer possible to fetch public sources, one has to skip all the enhancements appearing there.
This question applies to core developers as well 🙂 For me, it certainly needs some addiction and love for this blob of nice software.
... or worse, they post extended wishlists here, which they expect to be implemented for free 🙂 Some even yell around if they don't get donated what they want (looking at you, @Briljander).
And then there are merchants which got the idea: they engage developers for this or that task, happily pay an extra hour for solid solutions in favor of barely working ones and happily see the result of this work going public.
That said, one reason for not so much appearing in core is that most work on shops goes into third party modules. Some are quite fragile and break as soon as used together with another module.
Another reason is that new features aren't ready for prime time the moment they get implemented. Core Updater had to be written three times until it was good enough. Err, I mean until it was as great as it's now. Merchant's Edition's next new feature, recognition of updateable PrestaShop modules, currently gets implemented for the second time as well. First version simply didn't work out in some details, so it doesn't make sense to push these commits to public 🙂