There are a couple reasons. What I found when I owned my own modem was whenever there was a problem I first had to convince them the problem was on their end, since they could always shrug and say "it's probably your modem". They can connect to their own equipment and immediately identify the problem.

The other reason is you might not have the money when you sign up. For a lot of people it's easier to come up with $10 than (in Starlink's case) $500.