Long polling is more amenable than SSE for most HTTP tools out of the box, eg curl. The SSE message body is notably different from plain HTTP responses.

To the OP, you can still build APIs with long polling. They are uncommon because push patterns are difficult to design well, regardless of protocol (whether long-polling, SSE, websockets, etc).

Whiteboarding a push API is a good exercise. There is a lot of nuance that gets overlooked in discussions whenever these patterns come up.