Why did many Romantic poets express a dislike of cities?

Nature was a central topic for Romantic poets and they often contrasted the wild, untamed beauty of nature with the frequently abyssmal conditions in the cities. Industrialisation brought change and not all of it was positive. Factories offered work, however, they also created pollution and exploited their workers. Slums were common and disease, hunger, and poverty were a part of everday life. Take a look at the second verse of William Blake's poem London:
I wander thro' each charter'd street, Near where the charter'd Thames does flow. And mark in every face I meet Marks of weakness, marks of woe.
Blake describes London as a man-made, planned structure; even the river Thames, originally a part of the wild nature, has been tamed. The people he meets are worn out and unhappy, likely due to the issues mentioned before.