It's the tradeoff of having such low orbits that a StarLink customer might be closer to a satellite than a terrestrial ISP's nearest node. Lower orbit means less surface coverage, so you need more satellites. That's why old-school satellite internet providers put them 20,000+ miles out and have pings pushing or exceeding one second.
The 22,236 mile figure comes from the desire for a geosynchronous orbit. Not a desire for increased coverage.
Many satellites at that altitude use specialized antennas to limit their coverage to specific areas of the earth. There is no sense in wasting the little available energy on a satellite in sending RF to places where you don't intend to provide coverage.
It is just the tip of the iceberg. They have FCC approval and plans for nearly 12,000 (as of 2019), and are considering a possible later extension to 42,000.