Why do most western states have straight lines?

Answer 1

Many states in the Western US had their borders made along lines of latitude and longitude resulting in straight borders. This was done because the areas that became states were sparsely populated and often not entirely explored. So that a costly exploration and mapping of the borderlands to find a suitable boundary could be avoided, the people in charge of the states simply marked borders along lines they knew the existence of.

Answer 2

Most western states in the US were surveyed and divided into grids using the Public Land Survey System (PLSS) in the 19th century. This system was used to facilitate land sales and settlement, resulting in the characteristic straight lines and right angles seen in the state boundaries and land plots.