Why does everything suddenly always have to go too fast? Well, everything is always going faster - but a thorough preliminary check should still be possible.
Because if it isn't released "quickly" people complain with "What's taking so long", "When will 3.3.1 come out" etc, etc, ad nauseum. Pretty sure you were aware there was a RC version as I see you are on area51. 3.3.1 was released due to a security vulnerability which, IMHO, takes precedence in fixing than ensuring a drop down menu works correctly (which as far as I'm aware was found out by someone viewing the ACP on their phone.
Isn't it common practice not to validate a FINAL version before everything has been checked - to submit it to the responsible developers and team members for review and validation before release?
It, normally, isn't the developers duties to test changes. I highly doubt if every single thing can be completely vetted by a testing team. Things are bound to get missed. Saying that, the page could have been posted to W3c validator which would have shown the error rather quickly but it still wouldn't have been fixed prior to 3.3.4. Just like the PM issue that was found wasn't immediately released as another version.
There are close to 640 folders in a download of phpBB. Even some of the third party libraries in use have run into things not working correctly.
Or have the validations and structures been changed in such a way that such and/or other types of errors go unnoticed in the final versions?
No one is perfect and without fault. This is a minor snafu as opposed to a full blown security vulnerability and it's being taken WAY out of context IMHO.