Question

why do supply curves generally become less nearly vertical as the time period under consideration grows longer?

Answer 1

The supply curve becomes less vertical when the time period under construction increase because in a long period of time the supply of the building can be increased. And the supply of building in the long time period becomes more elastic. As it get more and more elastic the curve will get more to the horizontal and less vertical.

In the short run because the supply is inelastic i.e. if the price increase the supply can't be increased. it will be more vertical.