But faster run-off sounds like a good thing to prevent the flood - are you saying that more development upstream means more flooding downstream because all the run-off arrives with much lower latency and higher bandwidth, to use IT terms?
Also people tend to drain marshes and any other shallow stagnant waterways to avoid mosquitoes having breeding places. It can save us from terrible diseases, but it can make flooding worse.
When Rainfall * Area >= SoilAbsorption(See note) + WetlandAbsorption + ArtificialDrainage the end result is flooding.
Essentially the left hand side of the inequality has been slightly increasing because of climate change, but we're chipping away constantly at the right side of the equation in three ways.
(1) More and more soils in American metro areas have been covered by impervious surfaces for development. (2) More wetlands are being destroyed for development. (3) Artificial drainage is being under-built. Drainage is generally built for 1 in 100 year events but the rate of urbanization is increasing the magnitude of flooding. (Drainage is only a problem for the developer that hasn't sold what they're building.)
Note: Soil has a finite capacity for water infiltration.
The tables on the wiki pages shows "curve numbers" for different types of soils. These numbers are based on experimental data which is then used to model runoff. (Or used to be. Its been 20 years since I've done runoff modeling, so maybe things have changed)
That probably has a small reason to do with the increase, but your next line highlights the issue.
> or that we're noticing the floods more now because there's more damage to be caused to more things that have been built up
No, not exactly. The more stuff you build, the faster water runs off. Unincorporated land has a pretty high water slowing capability. It takes more time for water to run through tall grass, and puddles, and log jams, and loose dirt. All things that occur naturally. Humans build houses that have all the water run into gutters, that run into a well cut lawn and down a sidewalk. Once it hits the road it's like a water expressway. It can travel quickly to a local stream. A stream that likely has a concrete lining so the water moves swiftly to a lake or reservoir in hours rather than days.
As a place develops new flood plans must be studied every 5 to 10 years. A new development of 100 hours up the watershed from you can mean your risk of flooding increases dramatically. Places like Houston have had 10,000's of houses build in the surrounding areas meaning what was a 1 in 100 year flood could be as low as a 1 in 10 year flood. All this stuff being built is not taking into account its effect on downstream rainfall runoff.