Because you don’t control my domain. Suppose I own joespizza.com and you want to attack it using a supposedly legit load-testing service. You would go to the se

Because you don’t control my domain. Suppose I own joespizza.com and you want to attack it using a supposedly legit load-testing service. You would go to the service, sign up, enter joespizza.com/order as the page you want to test, and then be given a random string to add to a TXT record on joespizza.com. You don’t own joespizza.com, and you haven’t compromised my hosting service account, so you can’t create a legit DNS record. The service refuses to stress test my site, and you move on to the next thing. How else do you imagine this working?