Why? It will take less than two years, maybe one year subscription to pay for the dish, and calculation show $10 a month pays for the satellites. It looks like pure profit for any customer who uses their service for three or more years, and there are a lot of customers out there.
It's not pure profit because, a) you can only oversubscribe 20GB/s so much without data caps and speed limits b) a small fraction of the constellation will be over inhabited land and wealthy enough countries at any moment c) I am assuming they lose money the first ~1.5 years to recoup the dish cost and R&D, d) ground link opex e) sats fall out of the sky and need replacing f) subsidized launches via rideshare only work if there is enough demand into inclined LEO, otherwise its $30M per launch at per sat costs go way up g) Customers are not uniformly distributed over earth's landmasses