You might grumble about $30/mo for something like Postman, and it's true that "back in the day" it might have been a $40 one-off, but that's closer to $90 with inflation now, and there's a good chance that would have only bought you version 4.
Then next year version 5 comes out, and you face a dilemma. Do you pay all over again? Do you end up staying on version 4 until eventually there's a compelling reason to upgrade?
SaaS solves that problem by keeping everyone on the latest version.
Software is economically expensive to produce, we don't do enough to recognise that, in part because of how much free work is contributed by open source contributors and how little we recognise how much work they really do.