First, is population density. You didn't say where exactly, but for example here in Tampa, Wells Fargo has 25 branches in the area (though that is a bit larger then what I would think of the Tampa area as a local)
Second, we can mix in service expectation. I expect that in addition to "good" online service, "great" phone service, "great" email service, that when I have a problem, don't understand something, or want to talk about my options for investing or choosing account types, that I am able to go into a branch. That I can "walk in" and see someone quickly, or schedule an appointment and see some one right away (at my appointment time).
Together, these two options means that on a busy day, the nearest Wells Fargo Branch to me has at any one time, 50 - 60 people in it. Smaller branches, of course have less, and larger branches exist. So it just takes that many branches to address the number of people and their expected needs.
As to why there are so many different brands/banks
Well that's just the USA. We believe in capitalism. We have believed in it much stronger in the past, but banks are the central to capitalism so why shouldn't they serve as an example.
At it's core (a very simplistic look) Capitalism and a free market means that we as customers are better served by having lots of different brands fighting for our business. It should drive more consumer desired features (like lower prices, higher interest rates, better fee schedules, etc.) while forcing those brands to operate "better". (Just ignore the bail out, that's a loaded topic)
So for some of us, we want a big bank like Wells Fargo, because we want the rates, structure, and service they can provide as a "big bank". For others they want the more personal touch of a "small bank". There are benefits both ways.
For example there may be a bank that only allows people with excellent credit to open accounts. That allows they to have lower over all mortgage rates, but means their checking accounts have higher minimums. While the next bank may be more inclusive, and have smaller minimum balances, but as a result charge more for loans.
We like our options, and rest assured all those "brands" offer products that have differences that attract customers.