Because more competition in the ad space is invariably better for consumers. The alleged practices at play are illegal because they are known to harm consumers. And this is sufficient to demonstrate harm. For example, collusion is illegal not because someone necessarily sues and says “I was harmed by this” but because collusion invariably harms consumers. That’s the whole motivation behind collusion/price setting. Seems clear as day to me though I inquired with my friend who works in antitrust litigation and this was the response:

“There’s the broader principal that if they’re setting prices illegally they may not be harming consumers right now but they certainly could. Second, consumers are hurt when innovation/competition is curbed, and that is what they allege the deal’s purpose is. The bottom line is that if there were illegal deals and they can prove there were, those agreements are per se violations and you’re not really looking at consumer harm. This is what the case is alleging.”

The brief actually sums this up really well. The claims start at p. 100, which is where they allege harms but they provide good explanations throughout.

Edit: Note about the the brief.