in movies or tv shows they blank out the middle finger? why do they do that?

Answer 1

Because the FCC which is the regulations people for tv and movies tell the channels what they can show and what they cannot. Regular channels cannot show such things basically, because it's the hand-equivalent of the f word, which is an no-no on the air too. Altho' cable pay channels like HBO and Showtime can get away with a bit more naughty behavior and language because the general public cannot view it without paying for cable first.
Example: The difference in regular tv channels and pay channels is why the series"Sex and the City" had to be cut and edited from the original HBO form before being sold for syndication (repeats) and shown on regular tv. They cleaned up the language a bit and a few of the more adult scenes to fly thru sensoring on regular tv channels.

Answer 2

its either the station or the censors blinking out the naughty stuff.