The issue I have with the "D&D as DIY hobby" viewpoint approach is this:
Why would I ever DIY for my home game using D&D as the foundation, when FATE and Cortex are right there and come with much less baggage to unpack and discard?
If I was in the business of producing RPGs commercially, then I could see the argument for making a 5e hack along the lines of AiME, Esper Genesis, Five Torches Deep, Pugmire, etc. Alternatively, I could try and make something new out of the rules chassis of earlier editions; Pathfinder did it with 3.5, Lancer did it with 4e, and the OSR market is (over)saturated with B/X and AD&D clones and hacks. But if you aren't interested in going so far as to creating a new game entirely (and possibly monetizing it), then I don't see trying to make 5e work for purposes beyond its RAW state worth the effort.
In the context of the 90s and the early 00s, when the Internet wasn't as developed as it is today and both games themselves and in-depth discussions on games theory and design wasn't as accessible, I can understand why DIY became the dominant ethos. But the field has shifted, the breadth of tabletop games has massively expanded, and I am very tempted to say that the community overall has a better understanding of the hows and whys of the ways tabletop RPGs function.
In that light, I feel that there's two ways you can go with DIYing 5e and have it be worth the time investment: you either make some very minor aesthetic changes, or commit to really taking it apart and putting it back together in a standalone game. If you don't commit to either extreme, you'll more than likely find yourself in some weird limbo of messy and clunky design that wasn't really worth the effort and probably would have been better off just using another game for your purposes, whether generic or specific.
I bolded the issue; the issue that you have is simple; a failure of empathy. As in understanding and vicariously experiencing the feelings and experiences of others.
You might never DIY D&D. That's fine! You are you, and that's a respectable choice to make. On the other hand, other people are not you. They are not interested in monetizing the game commercially. And they might not want to run FATE or Cortex.
So try and imagine that you are someone who does enjoy DIY, and does enjoy D&D, and does enjoy the two things combined! What might be some of those reasons? I will give you a few.
1. A long history to build upon. Imagine there is a TTRPG that has been around since the dawn of pre-history; since TTRPGs were invented. This would allow for you to easily "plug & play" elements of the past into the current game.
2. Ease of use/ familiarity. Whether you believe the 10,000 hours thing or not, if you have significant time invested in the "D&D" style of play, it can become increasingly easy to run the game, alter the game, and modify the game as you need to. Some might call this "path dependency" but the basic idea is that it the system and lore is second-nature to you, and therefore easily hackable.
3. The 800lb gorilla in the room. Lots of people play FATE and Cortex. Which is great! But everyone plays D&D. Is that an exaggeration? I don't know. I mean, I'm hard pressed to think of anyone I know or have met that plays TTRPGs that has never, ever played D&D (a version of it, a clone of it, PF, etc.).* That means that there is a massive, ungodly amount of 3PP that is churned out that is compatible with D&D. If you stretch back to prior editions, and prior 3PP, the amount of stuff you have to draw upon is truly insane- too much for a mortal mind (MUAHAHAHA!). This allows for a ton of great material that you can alter and incorporate into your own setting.
4. Experience. Building on (2) and (3), above, many people have experience running bespoke versions of D&D, and know what they want. Players are familiar with D&D. It's almost always easier for some of us to both create a custom D&D game, and to sell it to players that are familiar with it, than to create a new game with a new system and sell it to players.
Again, this is not to denigrate any other TTRPG; many are designed incredibly well, and are amazing at what they do! More power to them. But if you truly can't envision why people "DIY" D&D, then I think you aren't listening to the people who are telling you that they are, in fact, doing that.
*Yes, there is always someone. I fully expect to hear from the, "I never played D&D. I have only ever played GURPS Autoduel," person any time now.