Games are tested with players throughout development, not as a final check. The reason is that a designer knows the game too well to experience it the way anyone else will.
The designer's knowledge is unrecoverable
Once someone understands a system, they cannot see it as opaque again. Every control, icon and rule is obvious to the person who chose it.
That makes internal judgment unreliable for anything about clarity, difficulty or pacing in the opening hours, which is precisely where players are lost.
Fresh testers restore the missing perspective, and their value declines with each session as they too become experienced. Studios keep recruiting new participants for exactly that reason.
Most findings are about confusion
Testing usually reveals that players did not understand what was being asked, rather than that they disliked the challenge. Those require completely different fixes.
A player stuck because a door is not visibly a door is not experiencing difficulty. They are experiencing a communication failure the designer cannot see.
Teams therefore watch where attention goes and where players hesitate, since the observable behavior is more reliable than what testers say afterward. Memory of a confusing moment fades faster than the confusion itself did.
Stated preferences and behavior diverge
Players routinely ask for changes that would damage the experience, such as removing friction that gives an accomplishment its weight.
Designers treat feedback as evidence of a problem rather than a specification for the solution, because the location of a complaint is more trustworthy than its proposed remedy.
The frequent pattern is that a complaint about one section is caused by something several minutes earlier that set the wrong expectation.
Different stages need different tests
Early testing uses rough prototypes to check whether a core action is enjoyable at all, before art or systems are built around it.
Later testing measures completion, drop-off points and time spent, on builds close enough to finished that the results transfer to release.
Running the wrong kind at the wrong time produces misleading results, since players judge unfinished art as if it were final.
Live games test continuously
Games that continue after release keep testing through telemetry, which reports what enormous numbers of players actually do rather than what a small group reports.
That data identifies where players stop and which options nobody chooses, and it supports controlled comparisons between variants.
It answers what is happening but not why, which is why studios running live games still put people in rooms and watch them play. Scale and explanation are supplied by different methods.