Why are there so many relays used in a car, instead of transistors?
Relays are much more stable temperature-wise: a sealed relay has essentially the same characteristics at -30°C and +70°C, both temperatures being common for cars. A transistor works quite differently at -30°C and +70°C, so the schematic has to be designed to account for those variations.

I once worked on a product with temperature range starting at -55°C, which used both relays and semiconductor devices. The funny part about the design was that below -20°C only the relay part was powered, which activated air heaters and would only switch on the semiconductor part once the temperature reached 0°C.

Relays also offer galvanic isolation, which effectively confines faults. Common failures like short circuits usually damage only one relay, whereas in transistor-based circuits several devices along the way would be affected. I bet people still want their car's motor running even when the air conditioner or a window lifter dies.