The easy reaction is, "How in the hell did they miss that?" but there's actually a good chance they didn't. It just doesn't matter -- at a certain point they have to release a game no matter how broken it is, otherwise development costs skyrocket beyond expected revenue (this is called the Assassin's Creed Unity Dilemma). As Corey explained, the testers are never happy about it -- but there's a separation between them and the people who actually make the game.
"The conversations are always in notes. Sometimes they're in a different country. You'd write your bug and they'd send it back as 'Won't fix.' So then you'd have to talk to your lead. 'Hey, they don't want to fix this, they're being idiots about it, can I please send it back?' It's just this really passive-aggressive back and forth, sometimes it's just arguing at a brick wall."
This can be particularly infuriating when you discover a bug that actually breaks gameplay -- such as, say, a weapon that makes a level pointlessly easy: "When I was working on Halo 3, my main level [ended] when you're getting back to the Flood's ship. They're really tough to kill, except if you had a flamethrower. When the literal first pixel of fire hit them they died instantly. It took about two months of back and forth along the lines of 'Dude, seriously, you just walk through the level.' For some reason they just didn't feel like fixing it. Those are the weird battles you have to fight."