A lot of it is due to lenses. Lenses contribute at least as much to image quality as camera bodies. Pros will often have tens if not hundreds of thousands of dollars invested in lenses, and glass doesn't go obsolete. But SLR lenses are only compatible with other SLRs with the same mount. So switching to mirrorless isn't just a matter of buying a Sony body instead of a Canon body the next time you want to upgrade. It's also a matter of replacing thousands and thousands of dollars of glass (and that's assuming there are equivalent replacements, which isn't always the case).
It is possible, but SLRs have a bigger flange distance to the sensor, because a mirror has to fit between the lens and the sensor. To be able to use SLR lenses with mirrorless cameras, the mirrorless camera needs to have the same flange distance as the SLR, which would make the camera body bigger. This kills one of the main mirrorless features.
While you can easily add the adapter, you are using the lens in a way it is not designed for. It will work for various definitions of work, and casual users might never even notice the difference. But it will never be equivalent to its native usecase.
Some random issues of the top of my head..
1) Tiny tolerance/alignment differences between the target mount and adapted mount can cause the lens to become decentered or be mounted at a tiny angle to the sensor plane.
2) special mirrorless camera features might not work as well e.g. eye AF in sony / object tracking / etc
3) possible autofocus hunting issues on outer focus points or autofocus performance will be impacted
4) pro level lenses like the canon 500 F/4 combined with a 1Dx type body unlocks a faster focusing mode which wont be available with an adapted lens.
> Tiny tolerance/alignment differences between the target mount and adapted mount can cause the lens to become decentered or be mounted at a tiny angle to the sensor plane.
It is certainly possible to make an accurate mount adapter; see the Pentax m42 → K mount adapter. The "problem" is people don't want to pay for a good one; the Pentax adapter costs about 2x as much as the adapters everyone seems to buy.
I have had mixed results with the Sigma MC-11 adapter. Some lenses worked OK, and some didn't. I don't have the setup or the resources to do a thorough analysis. I attributed them to the 'slop' in the system - From the focus motor to the mount to the adapter to the AF system and sensor. I was probably unlucky and the +/- tolerances didn't cancel each other out.
It is, but lens mounts are largely proprietary and SLR lenses are big. Some wide angle lenses are also heavily corrected in order to accomodate a swinging mirror. Even if you can fit them with an adapter that retains most features, you wouldn't want to lug around a heavier lens, when you could just sell it and buy the native equivalent, which in some cases even performs better due to physics. There is no point in using a SLR mount for mirrorless as it's oversized and will negate the cost and size advantages of shorter flange to sensor mirrorless designs.
For specialty lenses of various kinds, some people find it makes sense. But in general, as you say, there are compromises in using adapters in terms of function, size, etc. and usually it makes more sense to sell the existing lens and buy a native one.
It is possible since 2012 with the https://en.wikipedia.org/wiki/Canon_EOS_M . There is a first-party EF/EF-S to EF-M adapter, and all Canon DSLR lenses work on the EOS M series of cameras with full functionality (aperture, focus, IS, etc.) and zero caveats. I'm not sure what motivated your question.
