That's because picture taking was a rare event in people's lives in the 19th century. It was relatively expensive, there were few photographers, and you had to stay perfectly still for quite a while to get a proper photo. That's why these are all posed pictures as the technology of the time made it nearly impossible to take "action shots" due to exposure times.

Due to all of these factors, people tended to dress up in their Sunday best. If you notice the ladies on the horses in that picture are quite well dressed too.