COST Targets, Performance and Reliability must all be met.
PWM also has to be filtered with expensive chokes and caps so it does not interfere with your AM rural reception from radiation from under your seat which now acts like an antenna. 15A Chokes can add significant cost.
When we made 100k/mo seat heater electronics for Chrysler's Jeep Cherokee, the qualification process was paid by Chrystler which included an Omron Relay Engineer who flew from Japan to qualify our Assembly process. The final Qualifying. test was Chrysler test drivers taking 2 vehicles up to Churchill and back to Detroit.
THat's not all but it is safe to say , they were well tested for reliability and our cost for all the simple electronics, conformally coated was around $12 so adding a PWM and filter would be a significant difference. That was back in the 90's so it may not be the same now , but cost and quality is everything. MTBF is less important as it supplements the spare parts business after 10yrs.
Ignition wire has long been carbon wire with high resistance to avoid similar potential EMC issues.