Because they are created by design agencies who could be creating a 'brand identity' for anything - they are all working to the same current trends, so are likely to have similarlities to other brand identities produced around the same time, and in most cases there are probaby also budget limitations. In some cases they even use the current year's fashion palette (the TPE one did), which is likely to mean they look dated fairly quickly.
Plus the DfT has in recent years had something of an obsession with a white / light grey base colour which has influenced a number of liveries (both those under their direct management, and those franchised). Seen at its worst in the dreary 'East Coast' livery, and more recently in the slightly better LNER livery on the IETs, among others.
The more plain ones with a strong dark colour predominating seem to last best - of the current lot, GWR probably falls into that category, as does the main Scotrail livery (although possibly not the livery used for their HSTs).
Of the privatised rail companies, GNER is probably the one which put the most effort into creating its brand, including all ancillaries right down to bespoke crockery and glassware in the restaurant cars. The main livery, of dark blue with red detail and gold lettering, worked well.
