I only live in Metric-using country and I have no slightest idea how tall is five foot, whereas 155cm instantly comes to my mind as, ah, at around my eye level. It's all about familiarity.
It isn't, really. If asked about my approximate height, I'd say 'eins-achtzig' ie 1.8m (notice me omitting the unit), which in practice also has only a single significant digit (or 2 if you account for tall people) and to me isn't any more unwieldy than '6 foot'.