Because for the 5e-only player/GM (or 1e, or 2e, for all that), of which there are a great many, it obviates the need to learn a whole new system. (side effect: it also obviates the need to buy or otherwise acquire said new system)
And I'm not suggesting 5e itself (as in, the root game) has to change to suit any given set of preferences; rather, those with said preferences can change 5e through houserule, kitbash, social contract, and trial-and-error to suit those preferences, and due to its I-can-only-assume-intentional vagueness of design the system can handle it. Learning and adopting tweaks to a known system is light-years easier than learning a new one from scratch.
This is prone to a whole host of problems, not least of which is Ship of Theseus syndrome. People have enough trouble confusing various editions of D&D with their small to large differences. You go changing the periphery of the root game (whatever that is), people are going to have more confusion rather than less compared to systems they know from the get-go are distinct (although as we've seen time and again there's always baggage moving from system to system).
It's also a lot of work to kitbash. If you want an original magic system, you have to come up with it pretty much from scratch, except it has to work with the existing mechanisms of hit points, saving throws, etc. etc. Or is D&D's magic system part of the root game? If so, then D&D will never, ever fit my set of preferences.
If you want to have formalized faction relationships, you have to make that up.
If you want to do combat in any way other than round-by-round, turn-by-turn, hp-ablative hit-or-whiff mechanics...well I guess that is clearly part of the root game, so I guess that's out of consideration.
If you want to go even broader and do conflict rather than task resolution, or actions with complications/consequences rather than success/whiff, you have to mutate the core d20 mechanic almost beyond recognition.
If you want characters that aren't defined by strict classes and levels, again, is that part of the root game or not? What even constitutes the root game of 5e?
Now, there is a lot in 5e that can be tailored and customized to fit different allocations of authority in setting & narration, genre trappings, and the like, but the root game (such as I'd expect many to agree on) remains one of success/whiff task resolution, a heavy mechanical focus on round-by-round ablative combat, and pretty fixed classes...whose abilities are heavily focused on round-by-round ablative combat and whose advancement is pretty regimented. And there's a big, wide world of other game styles than that which no amount of kitbashing
around the periphery of that root will get you to.