I understand that but I mean why can’t I copy screenshot and paste the screenshot

If you’re specifically asking why you can link to a photo/video already posted to a site or social media but can’t upload something directly from your computer/console, I would presume it’s at least in part due to the relative ease of content moderation this setup allows. I wouldn’t discount the likelihood that the photo/video linking option has filters in place beyond the standard language filter to prevent people from linking to adult content or images from unmoderated hosting sites. By not allowing users to post content directly from their PC/console and limiting the scope of sites they can link to, that pushes the bulk of the onus of content moderation onto the sites on which linked pictures/videos are being hosted. The moderators of these forums can’t realistically be expected sit through and watch every video from start to finish just to make sure somebody didn’t slip something questionable in at the 55 minute mark of a one hour video before letting it be posted. (΄◉◞౪◟◉｀)