Embedding an external web page

I know I’ve asked this before, but I’m not understanding why I couldn’t embed or paste a web page into Write.as with a copy code from another web builder. For instance, this site allows for building “stories” or pages to connect with websites.: https://shorthand.com/.

I’m not technical, so I just thought I’d ask about the basic function since this works with most websites. I understand Write.as is unique and simple, which is why I like it. But wanting to ask.