Small companies doing big things

There are some differences between the US and England that are almost cliche. You can't walk into an American shop with an English accent without hearing "Oh, I love your accent" or "Is it weird to drive on the left?"
But some differences are more subtle. Some differences you can only really know about by spending a bit of time in the other country.

Supermarkets

In the UK, there are no small supermarket chains, and no small banks. In the USA, there are local supermarket chains which are really small. In New Hampshire, for example, there is a family-owned supermarket chain called "Market Basket" (which sounds less funny in an American accent) which has only 78 branches!
In England, everyone can name all the supermarkets and they are all near-enough nationwide.

Banks

The same is true of banks. If you visit England, you'll see all the banks on a couple of high streets. They are: Barclays, HSBC, Royal Bank of Scotland, (owner of Natwest and, until October 2015, Citizens Bank in the USA) Lloyds (owner of Halifax and Bank of Scotland), TSB and Santander.

Why?

But what really interests me is why these smaller chains exist. In the UK, if a minor chain still existed in the supermarket industry, Tesco, Sainsbury's or Morrisons would have purchased it by now to increase its footprint. And the other of the big-four supermarkets is owned by Walmart, so why wouldn't Walmart buy up some of its niggling competitors? But also, why hasn't a UK or EU giant (such as Carrefour) bought one of the small US chains? Buying Market Basket would be an easy way into the US market, for example.

What is it, then, about the US economy, that allows smaller chains to survive un-bought in an otherwise competitive market? Or, what is it about the UK economy that makes smaller supermarkets uncompetitive?