They probably could have sued 4can if they wanted.
But they understand that the audience of 4chan is much smaller than YT's, and brings incomparably less money to the 4chan that YT's audience brings to Google.
Please note, their failure to assert their rights does not void their rights. They are free to assert their rights where it makes financial sense. Let's remember that recording industry is not about music, it's about making money on selling music. They are where the money are, or where they think their money may be bleeding, or where a lawsuit can bring in more money. 4chan is not such a place, YT very much is.
But they understand that the audience of 4chan is much smaller than YT's, and brings incomparably less money to the 4chan that YT's audience brings to Google.
Please note, their failure to assert their rights does not void their rights. They are free to assert their rights where it makes financial sense. Let's remember that recording industry is not about music, it's about making money on selling music. They are where the money are, or where they think their money may be bleeding, or where a lawsuit can bring in more money. 4chan is not such a place, YT very much is.