I agree with Pablo...these are kind of dumb questions...kinda like "if we call then fingers, how come they never fing?"

I seriously hope you were joking, but assume you were not...Roads are not straight for several reasons....
1. You cant pave through everything - there are many things that haver to be avoided when paving roads such as forest, bodies of water,mountains ...the list goes on

2. A whole bunch of straight roads instead of a few winding and reconnecting roads would be alot more work...

3...4...5...6...7...8 ..well im not going to keep posting though Im sure others will, because there are several other reason..if you really cant figure it out... I can tell you some more