In general, a gameplay video would be either a partial copy or a derivative work, and in either case an infringement of created without permission. Such a video might be covered under fair use in US copyright law, particularly if made for the purpose of commentary on a game or instruction in how to play or design a game.

In general, fair use defense is more likely to succeed if only the minimum amount of the work required for the purpose is used, and sound tracks might not be required for such a purpose. Thus a maker of such a video might choose to omit the sound to improve the fair-use case.

Moreover, when the sound track contains licensed popular music it would be subject to a separate copyright, and many music publishers are notoriously litigious, so prudence would advise omitting the sound.

All that said, fair-use is a case-by-case determination, and if the makers of such videos have not been sued, they (and we) can only guess how a case would turn out. As to why game makers refrain from having such videos taken down (if they in fact do refrain) one can only speculate. Game publishers may consider the videos good advertising.

Questions based on the absence of legal proceedings are inherently speculative, unless a copyright holder has announced a policy of not taking action and the reasons for it.

In general, a gameplay video would be either a partial copy or a derivative work, and in either case an infringement if created without permission. Such a video might be covered under fair use in US copyright law, particularly if made for the purpose of commentary on a game or instruction in how to play or design a game.

In general, a fair use defense is more likely to succeed if only the minimum amount of the work required for the purpose is used, and sound tracks might not be required for such a purpose. Thus a maker of such a video might choose to omit the sound to improve the fair-use case.

Moreover, when the sound track contains licensed popular music it would be subject to a separate copyright, and many music publishers are notoriously litigious, so prudence would advise omitting the sound.

All that said, fair-use is a case-by-case determination, and if the makers of such videos have not been sued, they (and we) can only guess how a case would turn out. As to why game makers refrain from having such videos taken down (if they in fact do refrain) one can only speculate. Game publishers may consider the videos good advertising.

Questions based on the absence of legal proceedings are inherently speculative, unless a copyright holder has announced a policy of not taking action and the reasons for it.