You may trust them to produce legit software and have good intentions. But it's not really enough - It's hard to use reputation to divine someone's proficiency and diligence in operating a web server without getting it pwned and the published trojaned. Several dimensions of risk / trust are relevant here, and the distribution mechanism can address some of the opsec risks in a way that is transparent to the end user (you).

A platform like PyPI also increases chances of something like that getting detected after it happens, giving you a chance to react in a timely way.