Why is the chassis used as ground in automotive electrical circuits
There's not an electrical reason, but instead a weight reason. By using the existing metal structure as a ground, it effectively reduces the number of wires by around half, and therefore saving a great deal of weight. (For example, otherwise each tail light would have to have two wires instead of one.)
Remember too, that some electrical loads in an automobile use a lot of current. A starter motor, for example, very commonly uses 0 AWG wire which weighs about 0.5 kg/m.
Interestingly, although not your question, the choice of negative versus positive is entirely arbitrary. In fact, back in the 1960s, Volkswagen used a 6V "positive ground" system for the Beetle up to around 1967 when they finally changed to the 12V negative ground system that is standard today.
The more wires you use the more wire you have that can fail. The more wires you have that can fail, the more likely it is that something is going to stop working. There's really no way (as far as I know) for the chassis to fail electronically that won't also make things stop working for other, more important reasons, so by using the chassis you decrease the number of possible ways your car, or some component thereof, can stop working.
While the cogent other answers are correct, I should add the really really obvious answer, which given this is an engineering focused site, should have been mentioned by now.
The automotive industry is highly competitive, with razor thin margins and a demanding environment to operate in. And copper is very expensive.