Then it is like saying why do some hackers hack into websites and just deface it rather than steal anything
Corporate espionage. You take down a competitors website their customers get frustrated and look for someone else to provide them services and they find your company. Some companies do this. For certain businesses it can cost them millions if their website goes down from a attack even for a few minutes because of A) the lose of business during that time, B) they have to figure out what happened and C) this qualifies as a breach in security and if it is even remotely possible (Even 0.01%) that the attackers could have gotten their customers private data they must notify them and risk lawsuits and loss of business.
international laws that state that breaking the internet is illegal
You would essentially be DoSing the entire Internet. Countless organizations would lose billions of potential dollars. You would cause damage to any servers that rely on DNS to function properly.
Just take Facebook for example. If suddenly nobody could get on Facebook (average computer user doesn't know IP address of any Facebook servers), Facebook would lose tons of advertisement revenue. Which happens to be the majority of their revenue. Now multiply this across all social media, all web organizations, all companies with websites.
Not to mention these servers don't all sit in the US. Some countries could very well take this as an attack.
That is much much easier. And can be done with a single person. As opposed to this scenario, would need many people coordinating.
Once I hack a website and shelled it I can upload a Java Drive by to install malware or even Exploit Kit so it is actually not even hard and then steal the Databases and stuff. Simple it will help you during long term and it is simple to do.