PC have always about flexibility. choose the part base on your need and budget. this have also been one of the foundation of PC gaming. the thing with those "gaming" prebuilt they most often end up being more expensive than making your own.
They also come with other benefits like warranty and support for the entire system. I'd wager most of the time people don't build their own PC is because they lack the knowledge and skill to be their own technical support. Yes, building a PC is stupid easy, but good luck if something goes wrong.
At the very least, you get a PC that you know works when you push the power button. And if it doesn't, you can send it back and get one that does.
also when i said DIY it doesn't mean everything must be built on DIY part. many of the home/office pc come with decent CPU but they only use iGPU or some super low end discrete GPU like GT720. nvidia create the likes of GTX750ti, 1050Ti and GTX1650 to serve this market.
No, I understand this part. However, again, I'm under no impression that the number of people who go this route is anywhere close to the number of people who buy a system that suits their needs and runs it to the ground before buying another. Mostly again, because they have to be their own tech support which they don't want to be.
But again, you're free to show me data that says otherwise.