Mainstream TV's and monitors refresh at 60hz, this means that they display 60 frames per second, so 30/60fps fits nicely into that. If you were to display something in between it makes for slightly messy maths, which in turn leads to screen tearing and the like.
@LonDonE
"Yes you are right my friend, it is a sad world we live in today! Satanists everywhere, gays everywhere, paedophiles everywhere, war and death everywhere!"
Really dude? Gays are as bad as paedophiles and death? Man, get your priorities right.