Why shouldn’t I just go direct to my bank and negotiate with them?
Because an experienced mortgage broker has more negotiating power than a single person and is likely to get a better deal. You would also have to spend a lot of time researching all the options like interest rates and fees, from all the different lenders.
You have a lot of other decisions to make like where to buy, what to buy, organising the move, insurance etc and may not have the time or mental energy to get into negotiations with several lenders.
A mortgage broker is on YOUR team and will negotiate for your best interests armed with experience and knowledge.