Road are straight where possible, however they tend to go around obstacles like hills and mountains and swamps and rivers.

They also have to actually go where people want to go. They need to connect town and cities that people travel between and unless those were built in the very recent past, they are not on a straight line.

In some parts of the world current roads can follow roads that were laid down centuries or millennia ago.

Those roads were also built as straight as possible because people 2000 years ago also anted to get to their destination as fast as possible, but technology has changed since then quite a bit.

Roads always try to be as efficient as possible, but the balance of what is most efficient changes over the generations as building bridges and tunnels and moving large amounts of earth around gets easier and people travel faster on roads.

If engineers could build roads completely straight they would, the landscape just gets in the way.