Because...

• Universities might want to locally host a mirror in order to waste less bandwidth and provide faster downloads;
• Large companies usually like to host internal mirrors for the same reasons as above and also so they can audit and pick what packages will be available for their end users;
• Flathub is slow af for some people;
• Local country-specific mirrors are always faster;
• In some countries not everyone can access the official flathub;
• One might be dealing with airgapped networks and systems for sensitive work and you want to have ways for your end users to install flatpaks;
• Fastly, their CDN might go down at any point (like Cloudflare sometimes does) and you'll end up with nothing;
• Flathub itself may be subject to a cyberattack and their service might get crippled for a days or weeks and you'll have nothing as well;

For what's worth Debian archive repositories are about 5 TB and people actively mirror then in universities, companies, cloud providers etc.

The question here isn't "why would you" but rather "why would I be unable to do it". Their actively gatekeeping their repository in a futile attempt to be the single and central point of flatpak distribution - much like what Apple does with the App Store.