Why do most MEP companies buy through main contractors rather than buying directly from the supplier to save money?

Just like outsourcing their work to MEP consultant firm to go over the MEP drawings and designs rather hiring an MEP senior engineer.

1. manage work within stipulated timeline, which includes procurement of nuts and bolts also.   
2. payment terms 

To save time, to overcome tension to match the specification mention in tender and make strong relationship with main contractor.

Agreed to Mr.Alex answer

Thanks

           It is very critical point and could be emerged profoundly in Mega projects, it shows the significance of contracts management and how much we need to spread the culture of contracts management in GCC and how we need to change our mentality to be in parallel with advanced countries such as UK.

         Furthermore, dealing with main contractor rather than supplier in spite of sacrificing in limited benefits, it enables the contract manager to float the tender specifications and to achieve the required balance between benefits and risks. Risk management assessment on regular basis during the CLM, could clarify to the top management the whole picture through specifying the risk and leading an investigation for the root cause weather regularity, finance, currency, operation, legal… etc. then to give the required recommendation to the main responsible persons to mitigate the risk because we cannot remove it totally and as a consequence to avoid the contract collapse then dispute and court.

       The skilful contracts manager by using his insight and experience to point to the risk accurately through using visibility, controlling and compliance then he can in his assessment find the remedy as long as he is still dealing in a flexible way with the main contractor. Finally, at the end of the contract both parties are happy to achieve the curtail balance between the benefit and potential risk. Thank you for the important question!