"they do seem to excel at getting undeserved business from the government "
That's simply explained.
A system costing £10m/ year to support needs a company that can support a £10m contract, right?
Wrong. In HMG land if the system runs 10 years it needs a company that can support a £100m contract
Because we're the British Government, and we're special.
Multiply all numbers by 10 or 100 and you see why only "Special" contractors (like Capita, Thales, and the other Usual Suspects) can possibly be considered for this.
Bit like Carillion.