Why do people live in flood plains?

Various reasons:

because that's where you can get water

because that's where you can build a sanitation system and safer roads

because that's where you can grow crops

because big rivers have broad floodplains built over millenia, and there are parts that are unlikely ever to flood

because they put more faith in levees than they should have

because the poorer you are, the worse will be the quality of your homesite

because they didn't want to be hillbillies