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.

A car is (still, in the modern era) an electrically harsh environment. The "12V supply" is typically 13.5 to 15 V, and may occasionally spike to 80V or more. There may also be some high-frequency junk on the wires from the spark plugs.

Relays put up with that abuse somewhat better than transistors, at least for "a similar price point."

The voltage in a car is only about 12V, which means that even moderately powered components can draw large currents. The dash in my car is illuminated by four 12V 2W bulbs. They draw a current of 666mA just to light up the dash! If you look at the fuses for all the circuits in your car, even the smallest will be 5A. Most will be 10-20A range and some even more than that. The reason relays are so popular is because they are durable, have low contact resistance, and are (sometimes) cheaper than solid state components that can handle the same current. Many modern cars actually do use solid state relays but they come in the same type of brick packaging so as not to confuse any mechanics.