i think a lot of wc are scared of actually having to go out and get more work,
I wouldn't say that scares me, but I don't like having to repeat work, and getting new business that's been poached by someone undercutting would mean repeating work (getting new work).
It takes absolutely no brains to undercut, however and fortunately, in my experience, customers tend to stick with the devil they know, and price is just one of the variables that they factor in when changing their window cleaner.
Other factors are:
Will this new window cleaner be reliable?
Will this new window cleaner do a good job?
Will this new window cleaner somehow be a pain in the bottom?
I think these things are 'unknowable' till after the changeover, and that's what puts off some folk when switching window cleaners; so is it worth the risk for a few quid? Of course, for more prestigious contracts there's more scope for undercutting, but then there's health and safety issues to be considered.
Personally, and this is purely my opinion, undercutting isn't a good way to attract new business, and the only business you'll get is customers who're the type who know the price of everything, but the value of nothing.