Hello Tobias,
If you're referring to the option when scheduling meetings to allow host and participant video to be turned on (example below), then this option will only allow video to be turned on if the individual users joining also have the option turned on on their end.

Put another way, video being turned on by default upon joining a meeting is controlled by the participants themselves. If a participant has the "turn off my video when joining a meeting" option enabled on their end, then their video will not be turned on when they enter your room (despite your "participants video on" option being enabled). This is for the privacy of the users--if someone chooses not to have their video turned on by default, then Zoom allows that to be the case no matter which meeting they're joining.
The best way to deal with this is to ask participants to turn on their video after they join your meeting.