It is due in part to them owning a cheap launch platform. The world has not caught up yet to how cheap space launches are going to be. SpaceX knew that with their Falcon 9 reusable rockets that launch costs for them were going to go down by a factor of 10 and with their new SuperHeavy, by a factor of 1000. Think $30/kg to orbit instead of $10000/kg. 14 of the 23 rockets launched by SpaceX this year were for Starlink. They want to build up a culture and skills for rapid relaunch and building StarLink gives them a hopefully profitable way to drive that.

Phased array antennas are necessary for this to work (no tracking of the low satellites necessary), because one wants to have the satellites in very low orbits to achieve low latency. Geosynchronous satellites are very far away from earth so that speed of light gives unacceptable delays for things like gaming, chats, remote server admin, etc. Very low orbit can get short pings (in theory better than fiber because light moves faster in a vacuum than in glass). But very low orbit means one needs a lot of satellites to have wide coverage and no dropped connections. They are thinking around 12,000 in the first phase and around 42,000 in the second phase. Having so many satellites also gives the system much higher bandwidth as each satellite is limited in data transfer rates.

Plus SpaceX is looking for a cash cow to fund their ultimate goal: a cheap, extensive Mars transportation system to facilitate building a city on Mars.