I agree that's a bad idea to do so since security scanning is inherently a cat and mouse game.
Let's hypothetically say pypi did block upload on passing a security scan. The attacker now simply creates their own pypi test package ahead of time, uploads sample malicious payloads with additional layers of obfuscation until one passes the scan, and then uses that payload in the real attack.
Pypi would also probably open source any security scanning code it adds as part of upload (as it should), so the attacker could even just do it locally.
("slow is smooth, smooth is fast")