I don't quite understand your post, but is your question why don't the ISPs of the sources of the abusive traffic sort it out?

The distributed nature of DDoS means each participating host isn't sending that much traffic, and there are often tens or hundreds of thousands of participating hosts. An ISP should verify claims of abuse before cutting off customers, and since most of the customers are presumably unaware of what their systems are doing, there will be a lot of unhappy customers and then you've got to spend a lot of customer support time on helping them clean up their systems so they can get back online.

I spent a fair amount of time sending out abuse reports for phishing / malware senders about a decade ago, and most abuse reporting addresses are a black hole. Even if you do get to someone who will do something about abuse, they won't do it quickly.

There's be a few high profile longer term DDoS attacks lately, but when I was running infra that got a lot of stuff, it was mostly people kicking the tires on DDoS as a service offerings and most attacks were 90 seconds long ... there's no way I'm convincing an ISP to drop a pwned customer over that.

Starting from there, this DDoS sounds like layer 7 DDoS which is easy to track to the immediate senders, but a ton of DDoS is volumetric stuff, often volumetric reflection attacks where the senders spoof your address. If you're getting that, best you can do is get the reflectors kicked off (or cleaned up) ... tracing back to the sending hosts means getting a reflector (and their ISPs) engaged to do a lot of labor intensive work.

All of that investigation stuff takes qualified people lots of time, that's your money reason it doesn't happen.