you can swap monero to btc any time - where is the advantage?

Liquidity, speed, and cost but it's somewhat subjective and depends on what matters most to you. Personally, I'd rather be able to spend directly, without the trials and tribulations of offloading into something more widely accepted.

I would also add minimal barriers to entry and adoption. I want more people on Nostr and using the product. Its easier to relate to and explain Bitcoin.