Because the person who pushes the code to the public repo is not the same person who makes sure it isn't malicious. You have a review process. Nothin is stoppin

Because the person who pushes the code to the public repo is not the same person who makes sure it isn't malicious. You have a review process. Nothin is stopping me right now from creating a PyPI package called Django2.0 and having some poor souls download it. Or creating a tiny but useful utility, having it become popular, then introducing an update with a backdoor.