Because as a loophole, certain employers write in the contract that the employee is a worker, not an employee, and is entitled to ask a substitute to do the work. The employer drafts the contract, the employee has little chance to challenge it. Even if in practice the employee cannot ask someone else to do the work instead of himself, this prevents the employee from using discrimination legislation. Any employer who wishes may evade that legislation.
delete the word "personally" from the Equality Act 2010 s83 (2) (a). Currently it reads,
employment under a contract of employment, a contract of apprenticeship or a contract personally to do work;