Is it Illegal to Use Assets from Another Game for Research Purposes?

Just a though but why the need to use someone’s stuff to make your stuff?

Plenty of products available to build by proxy that is affordable or falls under CC-zero or MIT that is compatible with UE4 licensing requirements. Ripping content from some other game is not necessary and at the very least considered toxic.

The only thing I can think of as to reason is the idea that one can make a game for free?