They need so many satellites that using higher orbits would quickly guarantee Kessler syndrome. Instead they orbit low enough to require significant fuel simply to maintain orbit. These low orbits also allow individual Starlink satellites to be less than 1/10th the mass of normal communication satellites at ~260kg.
I know there are good reasons to be in LEO, but wouldn’t higher orbits mean a lower likelihood of collision simply due to the increased distances?
Satellite visualizations always make space appear more crowded than it really is.
Worse a sofa hit with another sofa at 5,000+ mph creates a cloud of debris that then endangers stuff across quite a lot of space.