Because many modellers and collectors nowadays discern finer details and do not find moulded on \ in detail acceptable. The move across the whole modelling fraternity is towards finer details out of the box.
Ive modelled subjects from warships in 1:96, 1:72, RC aircraft 1:6 and cars in 1:24 scale in my time and I want fine detail. Many 'old time' railway modellers may hanker after reasonably priced (who wouldnt?) good models but many others now want fine detail. That admittedly comes at a cost. Dont forget some people also collect fine scale models including locos just for display and never for running on a layout.
IMHO, most locos and rolling stock from 10+ year old moulds do not cut the mustard any more. Hornby Magazine did a short YT vid yesterday about the Hornby RailRoad Plus sound fitted Deltic. It might look 'good' to some but again, IMHO it looks toy like and a relic from the early 2000s if not before.
Ranges like RailRoad, RR Plus and EFE willfill a niche as others move towards finer detailed models.