I'm really familiar with this, because I'm a QA Manager at a software company.
This happens at virtually every single software company in the world:
1. Product Managers want to release on the date they promised, and pressure everyone to fix things as fast as possible.
2. Developers fix bugs
BUT INTRODUCE NEW ONES IN THE PROCESS. This is called regression.
3. QA has tested nearly everything once, but it is impossible to test everything again for every single change a developer makes.
4. Eventually the release date rolls around, and each release has some level of bug risk due to regression.
There are tons of variables that can account for buggy software being released:
1. Product Managers - This is probably the main reason. They always plan for a ton of features but never plan for the time it takes to test it and retest it. They set a hard release date, then when it comes time to test/retest, they say "RELEASE IT!". They often do this by simply marking P1 bugs as P2/3s.
2. Quality of developers - Good ones write their own tests and test before QA to ensure what they write is high quality. Bad ones don't even think about QA. Developers are the only people that create bugs
3. Management - If this guy was managing 100+ people, there is no way he can be responsible for this specific issue. He's probably got a layer of management below him, and another layer of QA people doing the actual testing. At that point, he's simply trusting that the work they are doing is valid. I guarantee you he isn't personally deep diving in each test case himself to ensure quality. If anything, fault him for his hiring ability.
The simple truth, all software is buggy to varying degrees. You can never know what you don't know, until you know it. Maybe they didnt have a test for this bug. Maybe they did, but it passed earlier, but then got broken again at some point. We don't know the situation.