Tesla and SpaceX required a lot of startup capital (relative to most other technology startups). Yes, their ideas are simple, but few people have the capital to implement those ideas.
Additionally, the existing players in the rocket market didn't have much incentive to innovate given how few players there were. Your guess is as good as mine on why the automobile industry didn't continue much EV research after the 90s.
Its the same reason why Kodak invented the first digital camera and didn't put it in the market. We see the same pattern over and over again and if you work at any of these huge corporations, you understand that the main duty of the executives is to protect their profit centers at all costs. Any product that is capable of cannibalizing on the profit centers will face a tough road to success and at the worst, get culled early.
Then a startup comes along, reinvents or polishes that product and is very successful. Eventually it turns into a BigCo. And the cycle repeats.
The main duty of executives is to increase shareholder value, but their personal incentives are generally misaligned so they
abuse profit centers and undermine company future to hit short term numbers and get the bonuses.
I think it's similar to companies needing a different kind of CEO to bootstrap a company than is needed to scale up and streamline one. It's also very difficult to know which area and how much to risk on something new when you already have a successful product.
Additionally, the existing players in the rocket market didn't have much incentive to innovate given how few players there were. Your guess is as good as mine on why the automobile industry didn't continue much EV research after the 90s.