Sometimes you can. But there are also shows where the only (legal) way to watch it is on a particular streaming platform where it is "exclusive".

You mean the "license while they feel like it" kind of purchase?

If I could pay for individual TV shows and actually own them I'd definitely prefer that over the disaster we have today. Buying a blue-ray and ripping it is not very practical and it's by design.

Netflix (notoriously) does not license most of their content this way. You can't rent/buy Stranger Things on Apple TV, no matter how much you're willing to pay. If Netflix acquires Warner Bros, I expect this restriction to extend to that content too over time.