Do you remember the Fallout 3 fan remake on the Fallout 4 engine? The very one that was called Capital Wasteland — unfortunately, despite all the potential, the team stopped working on this global mod. The main reason is the possible legal problems that can arise when using voice from Fallout 3.
The head of the project explained in an open letter that without the cult voices of Fallout 3, a fan remake will not be what it should be. At the same time, he noted that if in the future they get permission to use voice from Bethesda and Zenimax, then the team will continue to work on the mod.
A similar problem was encountered and the team Fallout 4: New Vegas. They developed in the re-creation of New Vegas on the engine of Fallout 4. However, their solution is to create their own voice acting.