The problem is that those are treated almost like an app, you need a $99/year developer certificate to publish them.

Many third party ticketing solutions venues and events use do support this, but for instance if you want to sell tickets for a party and self-host, you need another external integration, or a developer account. Generating a PDF with a QR code, and publishing an .ics file is essentially free.

As alluded to in the ancestor comment, signing the .pkpass requires an Apple developer account.