While the lack of safety net and fear of being fired are all valid reasons for Americans not taking vacation, what about another reason- they don't know how to vacation.

From personal experience, I get twitchy just from being sick/away from work for more than a day. I only take one day at a time of vacation (usually on Friday's to stretch out the weekend, and usually to get extra studying done or run some errands) only to feel like I wasted a day. It could be cultural, my family never took many family vacations (maybe 1 or 2). I'm lucky that my work encourages taking vacations, but the idea of doing so seems so weird for me.