No. It’s because it’s practically a desert and deserts have problems with flash flooding since the ground can’t soak up water effectively. Throwing concrete on huge portions of the city also contributes to flooding issues.

My understanding is that the channels, especially channelizing the LA River, were done to handle floodwater from further inland in the watershed, especially the mountains in spring. Having to handle both incoming floodwaters and record-setting local rain is beyond their design specs.