well the tendency why most game developers use the mouse to fire weapons that don't need aiming is as simple as the control making sense
to others for example generally w , a, s and d are used for movement and spacebar for jumping since it makes sense but in platformers it's another story since a and s are used for movement but buttons like w and s don't make a lot of sense for forward and backward movement
so they are used for jumping and falling at the same time space bar is used for jumping since it makes sense but then comes shooting the only buttons that make sense for shooting are the space bar and the left mouse button , the spacebar already reserved for jumping leaves only one choice the left mouse button , but you may see in some games where jumping is not present game developers use space bar at that scenario