Management is certainly a kind of labour and even a valuable one. A good manager can ensure the efficient operation of a whole organization, and a bad one can waste the efforts of whole teams and sink entire departments. No one is arguing managers and executives shouldn’t be justly compensated for their labour.
The problem comes when the compensation becomes detached from the labour. When a business produces a surplus and instead of that surplus going to the workers who produced the surplus, it goes to the investors.
Often these investors perhaps put some money into the business many years or even generations earlier, and yet are still entitled to receive cheques every quarter despite not having any role to play in producing that quarter’s surplus. How is that just?