It's not that complicated. Humans tend to build in flat areas. Flat areas tend to be flood plains. When flood plains flood they leave behind sediment. There are additional ways that sediment builds up such as airborne deposits and importantly from growing plants leaving behind biomatter, but in a lot of cases flooding explains the substantial depth of sediment deposited.
It’s one of those overlooked facets of Nature. Much of the Great Plains sits on the Ogallala Aquifer. This is an underlying terrain, to depths of hundreds of meters, filled in by sediment from the Rockies (https://en.m.wikipedia.org/wiki/Ogallala_Aquifer#General_cha...).
The greater part of the LA Basin is a floodplain which consists of hundreds of feet of sediment from the San Gabriel mountains.
