It's the unfortunate truth that those famous celebrities bring in tons more public interest.
If you hear about an interview with Joe McShmuck, who voiced the main character in a new animated movie, you probably wouldn't really care. But if it's someone charismatic, loved, and well-known, like Keanu Reeves for example, suddenly a lot more people are willing to watch those promos. And a lot more will be sitting in their seats in the theater. Remember that announcement for Cyberpunk 2077, where Reeves was shown in the trailer? Interest and attention for the game skyrocketed.
Remember, at the end of the day, Hollywood movie producers care about one thing: money.