The problem with talking and arguing is that many people push/flood the discussion with low-quality arguments. This puts it back on the person trying have a good discussion to keep correcting or defending.

A public example of this is Trump who just overwhelms any discussion with bullshit. It's impossible to have a conversation where anyone learns something.

Writing lets all participants operate at their own pace, and the type of person who tries to overwhelm a discussion with bullshit often doesn't write very much.