I've always thought it was more a lack of confidence either in themselves or in what they are playing. Not learning songs properly and not being sure of what they should be playing but i guess that's more or less what you said Ghost Bass.
I occasionally look at my fretting hand when playing but i try not to as there's nothing worse than seeing bassists watching their neck all night. Maybe its because i don't play often enough these days altho in recent months i'm either practicing at home every night or rehearsing with 2 new bands.
Dave