every card has an equal chance of hitting the board, but cards outrank each other.

ignoring straights and flushes, i have AKo, you have 75o. both all-in

possibilities are:
a 7 comes, no A or K - you win
a 5 comes, no A or K - you win
a 7 comes AND and A or K comes - i win
a 5 falls, AND an A or K comes - i win
an A falls, no 7 or 5 - i win
a K fall, no 7 or 5 - i win

a very simplified example, but basically the ranking of cards means that even thoiugh your card is as likely to come out as mine is, on some occasions both ouf our cards will come, and i will still win

that in itself gives a large edge

also consider the "counterfeit" situation where your pair of 3's become useless because the board pairs higher than that. the other day i went all in with AK on a board of 443... my opp had 23 and called. I won because the turn and river were both Jacks, so his 3s were irrelevant.

Finally, not all hands go to all in situations, and the threat of higher ranked cards makes playing rags very difficult.