If you try to do everything by yourself from scratch then you are destined to stretch yourself too thin and fail. It's about specialization. Tesla was highly specialized in the manufacture of electric vehicles and batteries. They eventually branched into solar, which mostly didn't go so well. Now they're branching into chip design, AI, and robotics all at the same time with no prior advantage, and at the cost of their EV business which is not receiving any investment from their giant piles of money to fix quality control and service problems that have plagued them for years.