Security issues with generating passwords in Python?

I've read a few different posts about generating passwords programatically, and many have comments from people saying using standard random module is a security risk. Can someone explain why? If I am using the standard random library to select 20 random characters/symbols/numbers to generate a password, surely this is far stronger than many 'regular' passwords your mom or dad might have?

I understand that it's not true random, but I can't see how that's an issue in this use case.