The PHB needs to fulfill the function of providing inspiration for new players, and the backgrounds section use art and themes to offer baselines for those inspirations. Backgrounds need to be more than a quarter page of crunchy bits without inspiration, and the Core Player's Handbook is where these baselines belong. Having just the quarter page of Background rules removes any RP ideas and flavor, which abandons new players who are already intimidated by having to make blind choices.
Veteran players already know if their DM's campaign allows them to modify aspects of their origin, so it doesn't have to be written with them in mind. Also, backgrounds can vary based on the DM's chosen campaign setting (which I think we'll see more of in the future), and because the DM is the chooser of the campaign setting and knows what new or variant options make sense for their campaign, customizable rules are fine in the DMG.
When the DMG is out, I anticipate that the customizable backgrounds will be introduced in DDB, and this temporary lull between the PHB and DMG will be a memory. People will be able to use DDB to customize their backgrounds using 2024-only rules just fine.
Also, backgrounds are pretty cool to use as Level 0 NPC builders. An NPC criminal doesn't have to have rogue levels with weapon mastery and sneak attack, but they can have some skills and tool proficiencies that make sense for their background.