I simplified it a bit. Imgur and Reddit block embeds with CORS or something so you do need a deal.

For YouTube, you need to get a special embed URL, like https://youtube.com/embed/videoIDhere. That’s easy to generate but if your site includes embedded YouTube videos it also means visitors agree to their ToS, and Lemmy devs don’t want that. Believing in net neutrality, they would need to enable ALL iframe embeds from ALL websites, which could easily get messy with vulnerabilities, phishing and whatnot.