Effective DDoS is entirely about scale. That is to say, generating more traffic to bear against a target than it has resources to handle.
Simply put, individually, compromised servers and workstations cannot generate enough traffic towards the victim to cause any practical disruption. If they did, mitigation would be far simpler, and attackers would be quickly identified and dealt with. Blocking individual attacking IP addresses and reporting those instances to the authorities would alleviate the situation quickly.
Unfortunately, this isn’t the case. Instead, effective DDoS means disrupting a service, a business, or even a geographic region by orchestrating large numbers of compromised hosts and services in an effort to bring increasingly larger masses of traffic against a victim.
So how do attackers do this? Through a combination of means.