I'm not a very good speaker. I say "um" a lot. Sometimes I have to pause when I lose my train of thought. I wish I were a better speaker. But I don't wish I were a better speaker like I wish I were a better writer. What I really want is to have good ideas, and that's a much bigger part of being a good writer than being a good speaker.
Having good ideas is most of writing well. If you know what you're talking about, you can say it in the plainest words and you'll be perceived as having a good style. With speaking it's the opposite: having good ideas is an alarmingly small component of being a good speaker.
Being a really good speaker is not merely orthogonal to having good ideas, but in many ways pushes you in the opposite direction. For example, you’re not supposed to write out a speech before hand because it’s harder to engage with the audience, but then you don’t have time to think about how to say things well.
Any given person is dumber as a member of an audience than as a reader. You can only spend a short amount of time thinking about any given sentence, whereas when you read you could pause and take time to reflect. Plus a sort of mob mentality develops in a crowd where you’re influenced heavily by the reactions of those around you.
So writing is better than talks as a source of ideas. But talks are good because they let you see someone you admire in person, and mainly because they’re a good way to provoke emotional reactions and to motivate people.