Convenience is 99% of the answer.

Plus, when "renting" a movie costs $3.99, and "buying" it costs $5.99, there's not a particular reason to not click the "purchase" button.

I wish more platforms would let you rent for $4, and then show you an "upgrade to buy for $3" or similar for a week afterwards.

Why would they? They make more money if you pay the $7 up front.

Sure, but if it's buy now $6, or rent $4, upgrade to buy $3, they make a bit more the other way.

Depends on your habits. I don't re-watch most of the movie I see.