Same reason that anyone can contract with anyone to do almost anything.
When you go on holiday, you have a contract with a travel company - not the pilot of the plane - but you expect the travel firm to book you on an airline who employs qualified pilots. If you have private health insurance, the insurance company isn't a doctor.
The contract for provision of the service is separate from the competence of the operative that delivers it.
Think about the builder contracting to build the Olympic venue. Contract exists between builder and government for the whole stadium. The builder then subs out hundred of contracts to people like structural steelwork engineers, electricians, plumbers, concreters etc etc. They in turn either sub-contract or directly employ all the people who hold the relevant qualications to do the actual work.
There is no way that a single builder could possibly have all the relevant qualifications. His committment in his head-contract is that he will use qualified subbies for each part of the contract.
Ivors job is the same, just on a much smaller scale.
