Because to most people, a computer is like buying a car, it should just work.
A Mac is an Automatic, no configuration is needed outside of your favorite radio stations. Sure most people hate that the infotainment was replaced with a touch screen that only support carplay. But hey for the rest of the time they don't think about it. A widows PC is the same thing, but made by Tesla/BMW where the heated seats are a subscription service.
Linux is a range from manual to a kit car. Sure it can look like the big boys or even cooler. But the amount of work that's required is insane to the average user, and most people won't want to touch the hood, let alone to configure the infotainment so it can connect to your iPhone since it technically supports car play. But to those that know how to use it will swear that their manual car is better in every way than an automatic.