As a big buyer and someone well connected to authors on here - answers are:
Q1 - often personal circumstances, lack of time, unable to provide necessary support etc (rarely legal issues) and a lot changes in 3-4 years in web development.
Q2 - buy from very established authors helps but far from always any better than newbie ones.
Envato can’t allow updates or downloads post removal because they don’t own the files.
If envato adjusted their model around this then it would drive some authors away and have a negative impact on the wider marketplace. Even if buyer could still download the latest version there is no guarantee it will be compatible with the latest WP.
The bottom line is of corse it is frustrating if a file is removed but:
just like with anything in life you (to a point) will always get wage you pay for. A full WP theme like what we see here from an agency, which is likely to be more secure long term, would be thousands of $ let alone retainer fees to keep support and updates on going.
just like e.g. a TV - if the manufacturer removed a model then it may be hard to fix in the future and You would need to pay for an engineer to help you, as would be he case of you hired a developer to update and fix a theme with issues.
Getting something this valuable for $50 comes with considerations like authors removing files.
We’ve bought hundreds if not into the thousands of files and only around 10% have been removed so we’ve been lucky.
Hope that makes sense