Why are covalent bonds always sharing electrons in the outermost shell?

Because in the inner shells the electrons are already packed neatly  (eight to an orbital) and all the spare electrons are in the outer shell.

Its because those are the electrons that are unpaired...and its only the valence shell that isn't full. The inner shells already have as many electrons they need so there is no need for sharing.

think about this, in order to go onto the next valence shell, all of the eight electrons must filled in on atom therefore, the outermost shell is the only one that can participate in bonds ( because the other ones are already filled. think of it as an auditorium with rows of 8 seats . and the people wanting to sit down as other atoms trying to make bonds, they cannot sit in places already taken , they can only sit in the available spots, so if an outer shell only has 2 available seats, it can only make 2 bonds (either with two atoms, or with another bond that needs 2)