Or...

the original owner lost the Velcro that came with the bracket to hold the pump securely so added a zip tie, then the bolts fell out that were holding the bracket to the frame.
Or maybe the pump was purchased like that at a garage sale then fell out of the person's backpack before they got home and had a chance to attach it.

If improperly mounting a pump bracket is a disturbing level of incompetence, I suggest you not pay attention to bikes you see in the wild. A few Sunday afternoons in a bike shop will show you so many customers bikes with badly executed 'simple' repairs that your head will explode. Ever seen a person who carries their U-lock by locking it sloppily around the head tube so it dangles in the front wheel while riding? Ever seen an aluminum crank arm that someone has tried to weld to a steel bottom bracket spindle? Ever seen a broken frame 'repaired' with duct tape? How about a rear rack fastened with self tapping sheet metal screws driven into the M5 threaded mounting holes? How about a 'men's' bike modified to be a 'woman's' frame by hacksawing the top tube out?

A pump bracket falling off is nothing.