It's because they all belong to the same people
There's still a lot of independent ones or ones that belong to very small chains. And some of the ones that belong to larger chains do nice food.
controls 4,470 pubs.
That's the tradition. They once almost all belonged to breweries and were rented to the pub landlord (actually a tenant) who could decide whether to do food or not. Some of them did very nice food. In the Thatcher years, they limited breweries to owning a maximum of 2000 pubs and since then, most larger breweries have sold their real estate.
The availability of food from the large chains has spurred smaller ones on to doing food to compete and this has had a positive effect. Pub food is very often of a type; filling, traditional and easy to make. Some pubs specialise in traditional roast Sunday lunches. If you haven't had a Sinday lunch with all its trimmings I recommend it. They always go down well with Polish visitors.