Why there is so little liquidity in pre/post market (to the last moment)

Suppose market hours are:

7-9am (pre-market), 9-3pm (regular), 3-11pm (post-market).

I can understand there are less people trading at 11pm but what about 3:01pm? Why there is sudden decrease in liquidity at 3:01pm just 1 minute later? Why is not gradual loss of volume/liquidity between 3-11pm?

Same problem applies to 8:59am – 9am too.