In these large companies, to make changes to a lot of systems, you have to have someone else also sign on to the changes (eg. code review).
If your aim is sabotage, you would have to find a like minded saboteur to work with you. In normal times, at most places, this is risky, because most others would not want to assist you, and would likely report you.
The equation has changed. Musk has turned nearly every employee against him. It is now much less risky to explore your social graph now to find people who are of like mind. The defenses that software companies have are completely inadequate to deal with this sort of situation.