Fairly hard actually - you've got to stakeout the house without being detected and tail the guy when he leaves. Plus you may not know how many folks were in the house in the first place (and thus how many are still there when the main guy leaves) - you could wind up raiding a house full of awake and semi-alert criminals with guns and have a bloodbath on your hands.

Additionally, the idea of this sort of raid is to arrest anyone who might be involved when they're found to be in a drug house. If you raid an empty house and find drugs, the owner of said house is gonna go underground if he sees his place crawling with cops, and everyone else is gonna steer clear of it.

Finally, there's a time-sensitivity component - the entire premise of drug dealing is that the drugs are in more or less constant motion, so your chances of success go down the longer there is between tipoff and raid.

I'm not saying that I like no-knock warrants or that they're a good idea, I just wanted to offer some reasons when trying to hit the house when nobody's home might be less safe or less effective.