Money.

The general public thinks phones and computers are fundamentally different. Heck, I remember arguing this point even on HN back when smart phones were first coming out and being generally on the losing side as people got very excited about "app stores" and such. I see no practical path to getting to the point that enough of us realize that there is simply no reason for our phones to be locked down the way they are that the companies are forced to undo it, especially with our elites pushing with all they are worth to lock things down harder.

The companies take that confusion to the bank.

There have been numerous attempts at making phone/laptop crossovers, where you can plug your phone into a dock and get a computer, or slide your phone into a laptop case, etc. Some of them are even still around, but they're all definitely second-class citizens. There's a variety of problems that I think they've had in the market, not least of which is the fact that the average person still sees "phones" and "computers" as fundamentally different so the product makes no sense to them, but another issue that I think has held them back is that the product inevitably work by porting the limitations of the phone into the computer, rather than porting the freedom of the computer into the phone.

In the USB-C era, there is no excuse for every phone not having a mode where you can plug it into any ol' USB-C hub/dock and be able to get a desktop environment, even down to the "middle-of-the-line" phones. It would require in most cases no extra hardware. They just don't.

Money? You don't think Apple would make a killing on OSX licenses and lapdock sales if they allowed OSX on iPhone tomorrow?

Mac is a tiny slice of revenue for apple. OSX on iPhone would blow it out of the water. Apple would turn the PC market upside down, taking a sizeable chunk from Windows. As there'd be no point for most people to have a separate laptop/desktop at that point.

People also thought that phones needed keyboards before Apple showed them a better way. This is all on Apple to make a reality, no one else can bring general purpose computing to iPhone except them. It's their choice to make.