That type of computer you can buy already for years. Don't even have to turn the thing of even if you needed to change the CPU. Just push a few buttons on the front of the machine that mark which hardware unit required replacement, wait for the indicator's (led or display) "permission", replace the unit and push buttons in front again to mark the unit has been changed. All that is left to do is wait for "permission" to use the hardware fully again.
Those systems were not affordable by mere mortals. Not buying or maintaining the hardware...or licensing the specialized operating system(s) required by such type of computers.
Things are as they are, because any modular hardware design requires software builders to give a lot more thought and planning to handle hardware changes at any given moment.
Don't think for one second that any of the currently popular operating systems is capable of working with this kind of hardware. At least not without (very) serious rewrites in the best situation to starting from scratch.
Besides that, modularity usually means making use of connectors. Every time you add any connector, you weaken the structure of the electronic circuits. You also lengthen the circuit, introducing more heat generation and susceptibility to RF interference. These things inevitably result in slowing down your circuits, sometimes significantly.
These are only the (basic) technical limitations you will experience with modular hardware. Those technical limitations will already rob a lot "fun" from the idea, but that will be nothing compared to the legal limitations.
Back in the day where we still could "run with the dino's" on a daily basis (instead of that new age sissy and annual thing called: "running with the bulls"), I grew up with a Commodore 64 and at one point in time I was able to buy the Geos cartridge for that home computer. Suddenly my C64 had a complete GUI with a word processor, all kind of tools and the pointer could be managed by joystick, keyboard or mouse.
Still, the Geos cartridge was not that successful. At least I was the only person who actually had one and I went to a lot of places, meeting a lot of people, doing some serious "sneaker-netting" for years (30 to 25 years ago). Concept was there, affordable computers were there and even in those years the modular hardware system never caught on.