The last time I noticed this question (probably around python 2.4?), it was considered a deliberate decision.
There are languages with more open classes, such as (IIRC) Ruby and Smalltalk, but Python chose to remain somewhat closed, because monkey-patching is undesirable, and can be a problem for security audits.
Whether it would really open additional attack vectors or crash possibilities ... wasn't judged worth the time to analyze or the risk of being wrong. Maybe those tradeoffs have changed, but if you don't get any other answers, that (and the bias for status quo) is the likely explanation.