Well I know my players (and myself) pretty well, since we've been playing together a loooong time. Guilds/factions just don't do it for us. Oh don't get me wrong, running afoul (or with) a Thieves' Guild or being part of a family/clan when we're adventuring in Kara-Tur, etc. has its place...but it's typically dressing more than a front-and-center active element. It winds its way in and out of the narrative but it rarely becomes a major factor for any length of time (moments of decision/action, absolutely)...but it rarely stays in any truly focal element (we simply find them far too all-consuming beyond those moments). The game and narrative becomes "The Guild" instead of about the characters. We still include them, and they have their place in the character's lives and the narrative, but they aren't of particular importance to us. There have been moments and characters where membership in X was significant and played an important role in how that person RP'ed their character (especially in our Kara Tur games), but it's ultimately about the character, not the membership.I do agree with you. Unfortunately, some features of the adventures and world books are not reprinted in the splat books. A lot the new backgrounds were not. The haunted background we had in CoS was a beauty that many of my players loved and some of them use it even if CoS is long gone behind us. Just the treatment the guilds of Ravnica gave to (well...) guilds could be applied to factions and I used them to adjust what the factions were giving to their members in the FR and they became a feature that the players were now longing to belong to and not just some RP shenanigan that would only give something at the DM whims. The codified version that Ravnica brought was a spring board that made factions quite appealing.
I really think that each book can bring unique things to a DM's campaign. Whatever their experiences and tastes. But sticking to the splat books is not a bad idea either.
Backgrounds have never been an issue for us either. We just called them "backstories" before they were codified. So if none of the PHB or splat backgrounds fits the bill for a particular character, we just make one that fits by cherry picking elements from the others, or making entirely our own. Easy peasy.