What proof do they have that it was copied, other than "it looks similar to ours?" Multiple discovery happens all the time, and the patent process - for better or worse - is an effort for an inventor to officially put their "I made this" stamp on.
[0] https://torbjornludvigsen.com/blog/hangprinter-is-prior-art/
[a] what I would call "prior art", anyway. IMHO, IANAL (AFAIK)
For a patent to be granted, the implied argument is entirely different. For a patent to be granted, it must be both novel and non-obvious to somebody in the field. If a patent is granted that exactly describes something that already exists, then one of two things must be the case. If the patent description was derived from the existing work, it wasn't novel. If the patent description was derived independently, then it was the obvious solution to a problem. In either case, it fails at least one of the requirements for a patent.
Unfortunately, the patent office has frequently interpreted "novel" as "not covered by a previous patent", with the implication that every invention must have been patented at some point. The courts can go back and invalidate a patent, but that would be a defense against an actual lawsuit, and would carry significant risk to initiate.