Wouldn’t it be more efficient to make them in a more aerodynamic form, like cars?
Buses are heavy and slow (well slow on stop-start commuter runs), and the resulting fuel savings would probably be less than the revenue lost by the interior space you’d likely have to sacrifice.
Also, watching bus drivers negotiate those big vehicles in Sydney traffic (esp. pulling in and out of bus stops), they seem to need all the visibility they can get, which is likely why they’re right there at the front of the bus looking down onto the road. The hood and trunk of a car restrict visibility a bit - but in a car you can get away with it. A bus, not so much. I don’t think the drivers would like seeing the car in front at the traffic lights disappear under a big lump of streamlining.
Just my WAG.
I’ll just add that large flat panels are likely easier and cheaper to fabricate than curved shapes (at least if one is using sheet metal). Anyway, in my experience the boxier buses tend to be types that operate at fairly slow speeds within cities. Intercity buses do tend to have at least some streamlining.
Having written the specs for a fleet of buses that my former agency bought, I can say that all the above answers are correct. Aerodynamically efficient, yes, but passenger capacity-wise, no.
BTW, the disability-accessible door assembly on a GM bus in the early '80s had more than 25,000 parts! But I digress.
To see a truly streamlined bus design from 1930 go to
Why on earth would buses need to be aerodynamic? How fast do they ever go?
Plus, they have to accommodate a large number of people, some of whom are standing. They need all the headroom they can get; a curved ceiling would put some heads in jeopardy.
Because hip people drive sports cars. 
You do realize that there are lots of highway buses that travel between cities, right? Not all buses are intracity transit units.
Hip, inadequate people, you mean.
If you ever tried to jump the canal in Grand Theft Auto in a bus, you wouldn’t be asking that question.
You guys are so weird. (I was just musing this question yesterday).
How much of the aerodynamics of cars is actually about making them fuel-efficient at 60-80mph, anyway? I’d have thought it was heavily influenced by other things, such as a smooth ride (more important in a small aluminium car than a ten-ton bus), good looks, performance in crash tests (including pedestrian safety), and so on.
Trains also only seem to deserve streamlining when they’re doing 100mph-plus.
wind resistance isnt a huge factor til you hit 40mph or so. most buses who use the freeway still only hit them for short runs.
and one thing not mentioned is that streamlining a bus would create a front blind spot on a vehicle designed to move large numbers of people and be driven around pedestrians. not the best plan for safety
Many buses travel long distances on the interstate: http://www.greyhound.com/home.asp