If games are tested, why do they still ship with bugs?
Because games are among the most complex consumer software produced, the number of possible states is effectively unbounded, and testing has to stop at some point that is decided commercially rather than technically.
Why games are unusually hard to test. A game is a real-time simulation with physics, AI, animation, networking, audio and rendering all running concurrently and interacting. The tester's problem is combinatorial: player position, inventory, quest state, difficulty, hardware, settings, connection quality and timing all multiply together. You cannot enumerate the states, let alone test them.
Other factors specific to games:
Hardware fragmentation on PC. Thousands of combinations of GPU, driver version, CPU and peripheral, any of which can behave differently.
Emergent behaviour. Bugs arising from systems interacting in ways nobody designed. The more systems a game has — physics, AI, crafting — the more of these there are, and they cannot be predicted from the components.
Players do things testers do not. Millions of players will find, within hours, sequences no test plan would contain, and speedrunners actively hunt for them.
Content volume. An open-world game may have thousands of assets and hundreds of hours of content, all of which must be checked.
Late-arriving content. Testing depends on features existing, and features frequently arrive near the end of a schedule, compressing the window.
What QA actually is. A structured discipline — test plans, regression suites, automated soak and crash testing, telemetry, compatibility labs and certification against platform holders' technical requirements. It is not "playing the game", and the role is chronically undervalued and poorly paid relative to its difficulty, with QA frequently outsourced and among the first cut.
Why known bugs ship. Every project maintains a triaged bug list, and shipping means deciding which known issues are acceptable. Release dates are set by marketing windows, platform certification slots, retail logistics and financial reporting periods — and moving a date is enormously expensive.
Day-one patches exist because the disc must be manufactured weeks ahead while development continues.
Live-service games ship earlier deliberately, treating patching as routine — which shifts risk onto players.