7 lives/years: maintainability and significance of the number 7
Ajahn Brahm addresses this in his book Mindfulness, Bliss, And Beyond. He notes that most people are not correctly practicing mindfulness such that it will allow them to attain stream entry. 

Although this is an alluring promise from the Buddha, understanding it requires a measure of brutal personal honesty: Are we in-fact practicing true, unwavering mindfulness during all waking moments of our lives for seven days straight? Such a task is actually quite difficult; even when we get a good, mindful groove going, it is exceedingly easy to lapse out of mindfulness and have to start again. 

I imagine that much of the meditation average folks do is primarily aimed at cultivating (i.e. learning, developing, and strengthening) mindfulness. It takes full mastery of mindfulness to be able to apply it long enough to break down our delusions to attain enlightenment.  

One final point Ajahn Brahm notes: depending on an individuals level of attainment (i.e. once-returner, arahant, etc) it may be very difficult to truly identify them. Traditionally, only the Buddha himself could verify these attainments. Thus, we should be careful in assuming how likely or unlikely occurrences of enlightenment are, though it is clear that even basic stream-entry requires serious work.