Because the marshes have been drained and the city has built over them, they can no longer absorb the excess water. The flood events in Paris are only expected to become worse due to climate change.
In 2016 there was a huge flood. The Seine overflowed its banks by 6.1 meters (about 20 feet)! Even the Louvre (a famous art museum in Paris) had to evacuate some of its artwork from floors that could be reached by the rising waters.
One thing Paris is doing is trying to make the areas around the river a little bit more natural. This means replacing some of the man-made structures along the river with parks. That way, when the river swells, fields get flooded instead of streets. Also, the trees and other plants there help the ground act like a sponge to soak up excess water.
Climate change is a real thing for Paris, not just with the more-frequent flooding but also the intense heat in summer. Locals are now realizing that to adapt, they need to return the city to a more natural state.
