I expect that (a) bus companies get comparatively few requests for detailed fare information, other than directly to drivers at time of sale; (b) as with the railways, fare 'tables' are probably not produced anymore - all the information is contained within the database of electronic ticket machines and displayed on those machines when a Origin & Destination is selected; any printout of this information will likely not be in a format suitable for public use; and (c) the administration of bus companies are run on a shoestring - there would be an additional staff cost to keep such public facing information up-to-date, especially as there would probably be no simple link between the public information and what is contained in the electronic data sets to ensure accuracy.
I expect that bus companies are well aware that providing fare details would be a welcome (some might say essential) improvement to passenger information, but this has to be balanced with the cost benefit of providing such information. Indeed, some companies have gone some way down this road already. However, this has been full of pitfalls, and from this experience there will certainly be a thought of 'no information is better than wrong information'.
I expect that there is no simple program to be written and interfaced with journey planners - if there was it would have been written by now. The database will be a problem, and the system of 'stages' covering 100 times as many bus stops as there are railway stations, including multiple stops at the same location, and the fact that there never has been any kind of unified fare system, will all be issues. Plenty of work going on behind the scenes, and plenty more still to do!
Fares in this country are totally de-regulated, just like supermarket prices. There is no regulatory oversight by Traffic Commissioners or Local Authorities.