Because it doesn’t make the system any simpler.
Whether you encapsulate the traffic in SSL at Discourse, a reverse proxy, or via Cloudflare, Force_HTTPS needs to be enabled. In the scenario where the server talks to CF over :80 that means that any time you access the server directly (and bypass cloudflare) you’re using Discourse with Force_HTTPS enabled while HTTPS isn’t present.
Enabling HTTPs is easy, the only thing which defeats the process for CloudFlare users is the need for direct server communication during enrollment for the challenge to go through, which is also easy. It leaves the server in a consistent state whether the CF proxy is enabled, or not. There’s no inconsistency in behavior - which your suggestion would create.
And adding a reverse proxy is many orders of magnitude more complex.