They've done cook-books associated with various cooking shows. My guess why they don't do a more general cook book would be royalties issues; with a single show they only have to worry about cutting a deal with one or two celebrity chefs, usually as part of the contract to make the show, with a less specific book you might be looking paying royalties to a different chef for every recipe, without a prior contract, and that gets expensive fast.

Lots of other reasons; for example, there are still a lot of Fanny Craddock's recipes there, and she died in 1994 so you'd be looking at dealing with her estate - which is problematic because her rather odd marital history (see Wikipedia) makes it far from clear who is actually entitled to any royalties.