Why is dumbell bench better then barbell bench?

That's a matter of opinion. You can use a lot more weight with a barbell than you can with two dumbbells in any exercise. More weight = faster gains in size and strength. Also, getting two heavy dbs in position to bench (and putting them down after) can be a pain in the butt w/out a spotter. I'm not saying folks shouldn't practice the db bench, it's always good to mix things up.

Those are both factors.

However the biggest factor is not an obvious one. You push the bar with both arms, but do not have the same ammount of force divided between them. One arm may have 2/3 of the load and as such do that much more work than the other. If you split the bar into two dumbells both arms have to work independently to push the weight.

Yes and yes.