A bag contains 8 cards numbered 1 through 8 .THe cards are mixed thoroughly and two cards are selected at random . Find the probability that both numbers are even .

I know this should be done this way ,

P(both numbers even)=(4C2)/(8C2)=3/14

My question is why couldn't it be done this way :

P(even)=1/2 so P(both number even)=(1/2)(1/2)=1/4 ??