They may never ask them out, or put it off so long that by the time they do they're well into the Friend Zone.
Even with all of the above stacked against them, a shyer, inexperienced guy will sometimes have enough things go their way that they end up in a position where they're alone with a woman and want to 'make a move' on them (e.g., kiss them, put their arm around them, etc.).
Unsurprisingly, they're really nervous and hesitant about this as well. They may have a blatant, all-systems-go opportunity where the girl is clearly interested, but they can't manage to go through with it.
There they are, sitting on a couch watching a movie with their date when the credits start to roll.
He sees this, gets nervous, and never conjures up the guts to speak to her. He can't work up the nerve to talk to her another time.