Why do banks fluctuate the rmortgage rate even if its a fixed mortage?

Once a fixed rate mortgage loan is lock with the lender, it is locked, and never changes, So I assume you mean why do rates change daily? Mortgage rates, just like any other commodity, like stocks, can change based on many factors, and is really beyond the scope and space of this site to break it all down other than to say rates are tied closely to the movement of mortgage bonds. As bond prices go up and down, so do mortgage rates. Mortgage bond players are interested in earning as much as possible (yield), and consumers want a low rate, but yields need to be high enough to make the bond worth buying.