When you download the Netflix app, you only get a sign in button. The only extra info you get about how to get a Netflix account is a help button that calls support. I don’t know what Hulu does, but I presume something similar.
That's a good point. Most of this boils down to not having an upgrade nag prompt. Since netflix doesn't have a free tier, they are less affected. Spotify is in a tough spot because they want free users to upgrade.
They choose not to promote premium services whose payments are executed outside the Apple ecosystem, using (free) advertising channels within the Apple ecosystem.