Why A quarter wave on 75 meters is about 20 meters tall, so a mobile whip of 2-3 meters is a tiny fraction of a wavelength. Electrically short antennas have very low radiation resistance, and the loading coil, ground losses and vehicle body resistance eat most of the power as heat, so efficiency can be a few percent on the low bands. That antenna inefficiency, not the radio or the power feed, is what dominates HF mobile performance.
Watch out "Picket fencing" is the rapid flutter heard on VHF/UHF mobile signals as the vehicle moves through reflections, not an HF problem, and there is no FCC rule capping mobile output power on 75 meters.
Short antenna, short signal: mobile HF lives or dies on antenna efficiency.