I don’t think you’re wrong at all and I definitely don’t think you’re picking on them. I think part of why they do it is because of their image/reputation, but also some bands tend to work best with “stream of consciousness” kinda sessions and once you start trying to chop it up or edit things too much, the process starts to limit the creativity.
I have done some studio sessions with bands like that and it’s almost as if asking them to involve themselves in a more selective process somehow ruins their chemistry or the enjoyment for the production process and you end up getting no productivity rather than higher quality productivity. My guess is that they’re successful enough to have had folks around them see and suggest the same things you are and I would also bet that the reason it has kept going like this is because, for whatever reason(s), it doesn’t work for this group. I could be totally wrong though. 
As for The Clash, they are certainly one of the GOATs and you’re absolutely right about Sandinista. Some absolutely great material mixed with some junk that should have been cut and released years later on a b-sides and outtakes compilation, for sure.
