Earlier quoted context omitted.
We probably don't see AWS or Azure as source if these DDOS attacks because of egress costs.
People aren't paying with their own money for DDoS machines normally. Well, maybe there are some small operations like that. But often the traffic comes from a hacked service that's a part of a bigger botnet. It may lead to a larger detection ratio on AWS/GCP, but the attackers are not paying the costs.
Attackers aren't paying with their money, but it's important for them to go undetected. If your bandwidth is a fixed cost (ie isn't related to your actual usage) you are much less likely to keep an eye on it than if it costs an arm and a leg.