Because for International Business, a typical agency bulk contract ticket is a anywhere from $300 to $2,000 per person less than if you buy the same flights direct with DL.

For Intl longhaul coach, net/contract fares can be $80-$400 per ticket lower.

For shorthaul/within the US there are few contracts that offer a price advantage or any agency commission/compensation, so agencies will usually charge a booking fee.