Well some open access might as well indeed, XC however provide a useful route because they do under cut traveling via London in terms of time for most journeys
The devil in me
Edinburgh - Penzance direct about 10:45
Edinburgh - Penzance via London typical 11:00 although it can be done quicker than a lot of direct services at 10:42 including 1 hour allowance to cross London.
Why do people travel to a specific route?
Price so in the above example via London will normally win.
Speed - Little to chose in this example really.
Times - Little to chose really but it is not much good having a direct train at 1300 when you want to leave at 0900.
Comfort - subjective for me via London will win.
Facalities - Catering, Power points, internet etc again via London will win.
Window Gazing / Track bashing - not much difference.
Not wanting to drag bags around London, which may be override above it has to be XC.
In the case of Norwich - Liverpool you may enjoy crossing the pennies, but the over-ridding reason a lot of people would take the 158 over via London even if it cost more is so they don't need to use the tube / get a taxi to cross London.
Looking at Southbound only typically if you went via London you would leave at Liverpool at 1048 arrive at Norwich at 1552, just over 5 hours. Yes you may get an earlier train out of Liverpool Street, but would you risk it? If you go direct you leave Liverpool at 1052 and arrive at Norwich 1613, taking about 20 minutes longer. Many people will consider that the tube journey isnt worth the 20 minute saving. At least on the direct train that if you get a delay you should get to Norwich (yes it could be capped) when changing you could soon be sat waiting for a long time, people don't like this! I know Liverpool St - Norwich is every 30 minutes, but going the other way and you could be waiting 1 hour in Euston or doing lots of changes as you head to Crewe etc.