Not only are abbeys and cathedrals places of worship, but they are also in effect large cemeteries. These huge churches didn’t have graveyards; they WERE the graveyards. I was really struck by this on my first visit to England in 2004, when we visited the famed Westminster in London and the Salisbury cathedral.
At Bath Abbey, it is much the same.
The abbey is currently under repair. The graves underneath have settled, leaving huge voids and putting the floor in danger of collapse. Read more about that here.