There are a few reasons that I can see why they don't integrate the latest .net.

First is that the security model changed with .net 5. Next is that they subsume Mono/.net core into the foundation of the language and this cost them them the ability to support Windows native development, specifically anything to do with Win32 API.

If you look at .net 10 and compare that to .net 5 you can see that they are trying to reintegrate the Win32 API but now it is in the all new Microsoft namespace.

The amount of change is too significant to act as a drop in replacement for the original .net framework. Maybe they could have gone a side-by-side installation, but the rapid development of The NET Framework I think made it too hard to tie to an operating system update. They wanted to free it from that update cycle of once a year or every two years and allow the development to progress rapidly at the cost of having to download it and install it each time.