I suspect the information hasn't been presented properly. The umbrella company will be doing more than just making a margin on the work. They will be dealing with payroll and billing, insurance and offering the end client contingency if it doesn't work out. They may also be dealing with recruitment etc. in the first place. If the contractor doesn't like it he can set up his own and undercut the umbrella company, subject to any contractual commitments.
Contractors do get a funny idea of what they are worth. They are paid more, because they are temporary. They can usually deduct more expenses than those on PAYE and have the ability to push assets to their company and lease them to themselves or dispose of them to themselves cheap. Someone earning £182k will not pay more than 50% of their income in tax and NI. The contractor is confusing his billable rate with his worth. To know what he's worth, he looks for alternative employment.