When you are an adult not in school you probably don't need "all apps" and it is relatively inexpensive to get just the product you use.
Anyway, they are still around because they still have some of the best set of features, and are industry standards, though this may change in the future and in some areas is already in progress (and I welcome that! They need competition to push them)
The single time purchase also has the added benefit of letting me use that version however long I like. Personally I don’t need much of anything that’s been added since CS2, and as such a user I’d normally only be buying new versions of Photoshop when the one I own stops running on modern operating systems. It also means you’re not bombarded with UI shifting around for no good reason, some feature getting pushed in your face for the sake of some PM’s metrics, etc.
The only reason I even have a CC sub right now is because a credit card benefit essentially pays for it. If/when that benefit disappears so does my sub.
A big part of how they keep their relevance is people using those 'educational discounts' so that they are the tools that everyone learns to use in school, building up a moat against any alternative.
And 20$/m is not what I would call "very expensive" in the context of a professional product used by people and companies who make a profit from it. By comparison, Autocad and Revit are 350$/m each