They are dramatically less common here in Omaha than they were a few years ago.
We had a big local chain called Bucky's which sold Shell, BP, or Phillips 66 fuel, depending on location - they were acquired by Casey's.
And our Speedee Mart (another local chain) now sells Amoco fuel instead of the Shell they used to sell.
I think we have two total Shells in Omaha and its suburbs, total now (from dozens a few years ago), and there are a few across the river in Council Bluffs, IA. They are not common here or in much of the rest of the western midwest - MN, IA, SD, ND, KS, etc. and are far from being the default option here.
The density of Shell stations (along with Chevron etc.) compared to our unbranded equivalents here is something I've noticed when traveling to Texas, much of the south, etc.