I've noticed the same thing! Immediately after boarding, there's always a line in front of the lav, even before takeoff. I find it very annoying.
I think it has to do with planning. When I was growing up, my mother taught me how to plan ahead before going out (from what I see, it doesn't seem like many parents still teach this to their children). She taught me always to empty your bladder and bowels at home before going out, and to avoid public toilets as much as possible. That lesson has stuck with me into adulthood. Whenever I go out in public, I almost never need to use the WC. Whenever I fly, I NEVER need to go to the lav, as I've already prepared before boarding. As a matter of fact, I never get up, period, no matter how long the flight. As a matter of fact, I have no idea what an aircraft lav looks like. So far I've never had a problem, and the longest flight I've had was about 12 hours.
Of course there are those with illnesses that need to go often, like that nice old man that sat next to me once on a transatlantic flight, but for the life of me, I just can't understand why there's always such a long line before each lav. Doesn't anyone regulate themselves?