They do, they’re called QA Testers, Quality Assurance.
Thing is… and bear with me here… they’re HUMAN, and humans make mistakes, miss things etc.
It’s also not always QA, sometimes fixes get pulled last minute and will go in a later update. Nowadays updates are like busses, they’re scheduled at regular intervals (and yes, that can still mean they skip without releasing one) and you work to get whatever fixes you can into the next one before it ships, will never get fixes for all bugs into the next one, unless it’s a VERY small/simple project, not a video game.
If you wait to release an update that you are 100% sure does not have any bug in it… you likely spent years solely on that update and by the time it’s released, no one cares anymore as they’ve presumed your project is abandoned having never had an update for years it took.
However, for a more in-depth and industry-backed experienced answer, might I suggest you visit to https://askagamedev.tumblr.com and ask “Why Didn’t QA find This Specific Bug???” since Developers are contracted to non-disclosure for their own studio/projects.