The original goal was that Starlink would be SpaceX's cash cow. The demand for rocket launches is growing, but it can only grow so fast. If you've built the capability to launch so many satellites that you can't find enough customers for all your launches, one option is to simply find ways to launch your own revenue-generating payloads into orbit. That was the original goal of Starlink, though it seems to be failing at that goal.

Starlink is bankrolling starlink, it's expected to generate 12 billion In revenue this year with 2 billion of that being profit

Before it paid for itself spacex did and still does have a lot of very wealthy private investors willing to throw significant funds at the company