Smaller cars cost less to run, and Uber and Lyft offer an easy way for someone who has no qualifications for any other job, and those who may be highly qualified but speak little English, to make a living.
Drivers don't get paid extra for having cars which cost them more to run, so the majority use the moist economical cars which meet the minimum requirements which Uber and Lyft require.
If you want better cars and drivers, pay more for a proper taxi with an officially approved taxi driver. "You pays your money and you takes yer choice". Although in some parts of the world where there is little or no mandatory regulation of taxi drivers, even the official taxi drivers often have crappy little cars and speak little of the local language.