Because the way PHP work (1 process per request), it can't handle a lot of simultaneous requests and also anyone can crash ANY wordpress website with a single WRK command ...
If you have all the content cached by your CDN it might be ok, but if you have a dynamic website with API/DB, not just static content ... wordpress is slow as hell from my experience
I've been using WordPress for 4 years with good scaling and no downtime. It's only "slow as hell" if you don't actually know WordPress (which most WordPress users don't). ;)