I feel the primary reason is because things are updated so much more frequently now. It's probably not worth the time to fully document software only for it to become outdated shortly afterwards.

Whereas before, software came on CDs, and if you wanted to update you had to just buy the next version.

I regard it as short-term greed. Documentation and QA doesn't sell short-term; things like Sidecar do.

Perhaps the right way to tackle it is updating less frequently (feature-wise) and focus more on stability and documentation instead. Because good documentation is written by the engineers themselves, it means less output from them (it is a myth that you can just throw more money and human resources at the problem). Imagine, for example, macOS and iOS only being updated once every 2 years with a major release.