In SATCOM, the link budget determines everything. In the link budget equation is the gain of both the receive and transmit antenna. If your phone antenna has a small gain (for a cell phone, you can consider it 0dB gain), then it has to be made up somewhere, and in this case it's on the satellite. The newer Starlink satellites have higher gain antennas for this use case. Despite this, the throughput is still quite low. The link budget equation results typically in a CNR value which provides you a budget for actual bandwidth/throughput. With a 0 gain ground antenna, there is only so much a small form factor satellite antenna can make up for. The satellite antennas also are phased array antennas which allow for beam steering which is another key technology here. As the satellite rapidly moves over the land it has to be able to maintain high gain in the direction it needs. A fixed antenna would either have a high gain in a single direction requiring lots more satellites for full coverage or wide beamwidths resulting in lower antenna gains and overall lower bandwidth/throughput, potentially to the point of uselessness.

TLDR: Bigger antennas on satellite.