If that's the case why are XC services always rammed
It might be me, but times I have used XC, the trains have been rammed for parts of a journey and not the whole journey. This to me is normally down to the fact that the XC services in these situations are probably quicker services with less stops point to point.
For example, when travelling on XC from Bournemouth to Reading, the services when I have used them have been generally spacious until they have got to Basingstoke, then the service has been rammed until Reading especially if it was just a 4 carriage Voyager class 220. The service has also then been rammed from Reading to Birmingham New Street.
Now, the majority of that situation is down to the people needing to commute taking the fastest method possible, whether it is Basinsgstoke to Reading or with getting from Reading to the likes of OXford, Banbury, Leamington Spa, Coventry, Birmingham International and Birmingham New Street. You work out the travel time difference and the amount of different local services those commuters would need to use if they where not able to use the XC service.
For instance let's say that someone needed to get from Basingstoke to Birmingham. They could either travel into Vauxhall(London) or get the GWR stopping service to Reading, then travel on to London Paddington. But from both Vauxhall(London) and Paddington, they would have to get across London to Euston where they could travel with either Avanti or go slower route with LNWR. Alternatively, they could travel on the GWR service to Reading, then get a service to Oxford, to then get the GWR service to Banbury where they could change for the Chiltern Railways service to Birmingham Snow Hill. In my experience, most people just go for the easy option, which is 1 train A to B unless there is no alternative.
The problem lies withe the fact that that where XC runs the fast service, the DFT have not made provision to them to have enough in the way of stock on the services, to not only cope with the A to B traveller, but local demand in areas that the XC services travel through.