There's also the simple fact that Britain's railways were much more fragmented. Most Continental countries had either a single state administration, or a small number of large companies (eg 6 in France). Nationalisation/standardisation came much earlier in Germany and France. Prior to 1923 Britain had well over 20 main line railways operating their own systems (plus various minor railways) . Then they were amalgamated into 4 which produced a whole new range of designs , then they were nationalised, resulting in a new set of "Standard" designs...
Britains railways had many hundreds of different types of loco in the 50s. The LNER - just one of the 4 Groups - gave each class a designation based on letter (for wheel arrangement) and number. The J list alone (0-6-0, 0-6-0T) got to J94
If you want a RTR LNER 0-6-0 - the Southern Area alone has 3 leading candidates (J11, J15, J6) never mind all the lesser types. That's leaving out the NE Area and Scottish Area, which are separate debates....
Never in a month of Sundays can the RTR manufacturers cover more than a fraction of this. Hence kits are essential to fill in the gaps where sales volumes would be modest - to be truly representive/authentic you need the variety ....