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.