Incumbent automakers are cash cows, not startups. Tesla will spend all of it's money on the Gigafactory. GM will destroy new product lines that threaten it's current profits.

Remember, incumbent auto makers are not fast moving or innovative at all. They are all cash cows. Although they can predict that a move to EVs will keep them alive long-term, such a move will always be done with a small fraction of GM's resources and labour in the coming decades, and Tesla with Elon's burning desire to destroy them are probably going to win.

Remember, GM can't sell these two products on the same lot: One which is a cash cow, and one which only exists to combat some distant threat of GM becoming obsolete.

Also you're forgetting that Tesla cars are just better in every sense. Better brand, faster, longer range, novelty...