The obvious answer is that the more bugs you batch up, the higher the chances the next build fails - this is why CI became a thing, small iterative changes are safer and lead to greater throughput
True, but if each run of CI takes 6 weeks then you're going to vastly hamper development.
CI doesn't mean doing all the tests all the time though. The expensive tests still wait until there's a major reason to run them. I had the same question as the parent and I still don't quite see why this can't work.