It isn't just Capita
As others have mentioned, if the job requires less than one person, outsourcing makes sense.
Consider getting your car serviced - not many people diy, most take it to kwik fit. Consider having your teeth filled - not many people diy, most take it to a dentist
Following on. Many large cos have outsourced cleaners, but, Not many large cos outsource their accounting function.
So, outsourcing does work for non-core functions that can be done better by a more specialised group.
The thing is that with the rise of t'internet, that specialised knowledge is out there for anyone for find for not much.
This tells me that outsourcing should work where you employ less than one person (or in a complex area, where you cannot employ enough people to maintain the knowledge).
Beyond that it should work for a while, in new areas or areas where the business processes are changing, but once he work is well understood, and the processes change less, it will tend to work less well than in-sourcing.
This is why Jon Lewis (!) keeps on saying that Capita is an IT company and in the 'change' / 'transformation' business as that is where a viable outsourcing business _might_ be.