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.
Having an authenticated developer account at least provides some level of speed bump to scammers, and a better starting point for the police.
The charitable explanation is that the wallet is designed for credit cards, and tickets were an after thought. Though I suspect it is really Apple trying to keep a walled garden, just like they always have.
