You can use acrylic to make a tank.
In comparing your pricing, are you sure you were comparing apples to apples? That is, was it perhaps comparing 1/4" glass to, say, 1/8" acrylic (which probably wouldn't be strong enough)?
As to why tanks are usually glass, as guppylover said, glass tanks are much harder to scratch (although it's easier to remove scratches from acrylic). Also, I'd guess that if you're making tanks in volume, glass is probably less expensive overall. In addition, and don't quote me on this, but I seem to remember reading that while a glass tank can crack, acrylic tanks tend to "blow out" a panel on failure, leading to instant disaster.
Just for info, I've toyed with the idea of making my own tanks, and after pricing out material, I'm ready to accept the fairly common advice "if the tank is less than 150g, it's probably cheaper to buy it rather than make it". If your're talking about a "custom" shape, though, that's obviously not an option.
And, Doggfather, I'm not certain, but I am under the impression that for very large tanks (above 150-200g), acrylic is usually preferred (in the correct thickness), because with large tanks, the necessary glass is so thick that its weight becomes a problem, and the thickness distorts the view significantly.
Not sure, though.