system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How to gaming companies miss big bugs and glitches even after test runs?
gpt
The biggest part is simply scale.

Blockbuster games sell tens of millions of copies -- GTA 5 has sold something like 55,000,000. Say that the average person spends 25 hours on a game (for some games, this is a high guess, but for others it's very low -- people often spend 100+ hours on COD or GTA during their ownership). That's 1.3 billion hours. Say that your testing department has 15 employees and that they work for 1 year, full-time, testing your game. For every 1 hour you spent on testing, the customers spend 43,333 hours on playing. When so much time is spent doing stuff in the game, the customers are *bound* to encounter bugs or glitches the testers didn't.

This is especially true when games are very large or complex, and have a long list of possible situations. Even in 30K hours, the testers couldn't test *every single possibility* in the game. Did they test what happens if a car explodes while upside down on a rock at night in the rain while the player is in the middle of the bike-mounting animation? Did they test what happens if you're in the middle of reloading a gun when a building collapses in mission 17 when your hand is supposed to cover your face, resulting in a weird animation glitch? Did they accelerate to mass speed while manipulating the camera with the mouse on its highest sensitivity but the aiming inverted? There are some bugs that only happen if you did one thing in one part of the game and another in a separate part 10 hours later -- if they were testing chunk by chunk (the usual situation), maybe they never saw that combination. 

But even when they do catch things, they don't necessarily get fixed. Development time is finite, decisions are made about which things are important enough to spend it on. Super-glitchy games are usually games that were rushed to meet a tight deadline, or which had small budgets, meaning it just wasn't possible to spend valuable developer time fixing bugs over adding features that have more impact on sales. Then even when you do allocate time to fixing a bug, some are easier to fix than others. Some bugs exist because of tiny oversights in physics engines, and are very hard to track down. Sometimes, the bug exists in a licensed engine and the developers don't even have the ability to fix it, they have to get the engine's developer to try and fix it and that might not be done in time for the game's release.