Of course coordinates "can" be entered manually.
The issue is that there is no "second level protection".
In any process where there is risk involved, one tries to arrange the process so that the following step will expose an error in the previous step(s).
In this case, a kind-of suitable check might be to display the route (on a nice big GPS screen, or an MFD) and check that it looks right. Yet the kind of pilots who will be entering manual coordinates are those not likely to have a "decent" GPS.
In the airline world, the 2nd pilot is supposed to check the coordinates, which is why (in theory) the airways charts all have the lat/long printed next to every waypoint. How many people actually do this?? Every digit I mean.
The next Q is whether you would rely on such a waypoint for terrain clearance when flying in IMC, especially in a terminal area
