system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why some YouTube videos can be accessed via PC while the same website cannot be accessed via a mobile device.
gpt
Most of the time it is due to copyright. If a video contains an audio track YouTube thinks is copyrighted, it is blocked from mobile since the mobile player is not designed to show the advertisement ("Buy this track link").

Other times it is because when you upload a video you have an option as an uploader to block mobile devices.