I know a few people have pointed out that inaccessibility is probably the main issue contrary to what you’ve said, but I just want to highlight the fact that your problem seems to be that inland bodies of water don’t get observations of fish that aren’t easy to catch with a pole and the example you used to highlight that inaccessibility wasn’t the problem was pointing out that there are fish observations in places that have hundreds of guided scuba tours, places to rent scuba gear, and even walk-in shops where you can buy scuba gear, yet you seemed to have assumed that 1. most everyone who made those observations actually owned the equipment they were using and 2. most everyone who made those observations traveled from inland. I’m sure one of those two things is true for most every reef observations, but both of them being true seems like it would actually be particularly rare.
Unrelated to that, the Midwest in particular has a problem with blue-green algae from fertilizer and cow paddy run-off. This makes it not only dangerous to enter non-flowing bodies of water in the Midwest, but it’s also straight up prohibited in most public parks (at least in my state). The ocean isn’t really that prone to having an outbreak of concentrated poison because it’s so large, the poison will disperse, making the ocean more accessible most of the time