You have to understand that being a merit source is an unpaid position, and pretty much one with no rewards at all other than to individual members who receive merits from them and the forum as a whole in so far as account farming is essentially dead. 

Second, many sources didn't even apply for the position (like me).  Back in September 2019, Theymos sent out a PM to a number of people in which he announced that they were now merit sources.  Personally, I take the job seriously because I support the merit system 100% and want to see it functioning as intended.  That might not necessarily be true of other members who got that PM, or some of them could have dropped out for whatever reason.  The data on merit sources isn't exactly transparent.

So while it would be great if more sources busted their asses, it's a volunteer job and Theymos is very much a hands-off kind of boss in that he doesn't contact you to say you aren't giving out enough merits or the like (at least I haven't heard of him doing that; someone correct me if I'm wrong).  To offer a service like fillippone does, actmyname did, or I do can turn into a monumental pain in the ass.  Your inbox fills up faster than you can deal with them or you just get way more requests than you can handle if it's a thread.  I imagine a lot of sources just don't want to bother with that--and that's OK.  Everyone has their own particular style and methods in being a source.

However, I don't know how many merit sources there currently are, or how active they are, but I'll betcha it'd probably help things if Theymos added some new ones.  Speaking of that, I haven't seen any new applications to be a merit source in quite some time.