What is the problem with sub contractors not working under the main contractor?

A sub-contractor is a party which agrees to perform part or all of the obligations of another party (main contractor) under a separate contract (master contract) with the ultimate employer (employer).

The sub-contractor will usually be engaged by a main contractor to perform a specific task as part of the master contract.

if sub-contractor doesn't work under the main contractor may cause a Conflict/Contradict of responsibilities or consistency of workflow and implementation.

extreme shifting of risks to subcontractors