Because if someone has a better hand than you, you want to find out now, and not on a flop of 378. If you do not raise A2 suited then you lose a lot of value from all the chumps who are happy calling a raise preflop with A3 and in this case 2445. You lose a LOT more value by not raising, than you ever would from people with AAxx reraising and you folding. The biggest mistakes in Pot limit omaha/8 are ALWAYS made preflop in the long term.
A perfect example of this would be this hand I posted on my website
http://www.ribbo.com/omaha7.htm . I raise with the worst hand from 3 people who see a flop, but if I don't raise then I can't raise with A2 either. And if I don't raise A2, then I am only raising AA. And if I am only raising AA, then everyone else will fold to the rock garden in seat 3 who has only raising 1 hand all hour.