Just one typical user experience with Facebook on a desktop browser:
You can see how user intent is not the driver for the experience here.
Next time you think about how we got to this place, with people having the attention span of a fruit fly, remember how it was made this way by design.
Remember that Facebook is not just “a tech company”, as you often hear it referred to. Facebook is foremost an advertising company. Its whole business model is about selling ad space to companies, so they can book ads on Facebook that are specifically targeted towards you. To better tailor these ads to your interests, Facebook needs as much data as it can get from you – inside Facebook.
The ads are on Facebook too, so Facebook has no interest to have you share anything outside of Facebook.
There are ways around Facebook’s anti-user design. But these involve steps that are inconvenient – something Facebook is betting you don’t want to do. For instance, you can go to a search engine (preferably one that doesn’t track you for advertising) and look up that video, or other items you wanted to share.
Or, if you want to actually share the Facebook post but outside Facebook, you could look up the page’s code (using the page inspector) and find the link to the actual post.
Or you use the mobile Twitter app and choose “open in Safari”, or “copy link”. In any case, on the desktop experience you’re locked in more or less unless you become elaborate to work around the fence.