I’m not sure exactly what you mean; curved or sloped?

Either way, keeping a bridge straight and level is simplest. It distributes loads evenly on both sides, allows the builder to use straight and square members (rather than something that’s slightly off), and things like that. They can make bridges with significant slopes and curves; freeway interchanges are a good example. But they are more complicated. And if a bridge needs a slight slope, oftentimes it’s easier to simply fill in dirt on the low side to make it level rather than have a bridge compensate for being less balanced.