Check out their old blog post on how they got real-time collaborative editing without conflicts to work: https://www.figma.com/blog/how-figmas-multiplayer-technology...

Collaboration and the whole editor UI in general is a much more complex task than your average glorified PDF with some dynamically rendered data.

It's not the Pinnacle of complexity, just more complex than your average app.

I'd say the multi-user aspects and level of scale add quite a lot of complexity in to the mix.