The other reason for is that the developers of the USB-C specifications left us with a terrible confusing mess where visually identical cables have different capabilities and behaviors.
If it’s confusing, just don’t use it. They could have stuck with a micro-B charger, or a custom charger, and no one would have questioned it. Instead, they decided they were confident enough in their understanding of USB-C to implement it on their device. And they were wrong.
I don't think anyone would complain much if they supported a popular barrel connector with a popular power draw. As it stands, I'll only ever use a Pi with a charger sold as working with it after all the 3B+ issues I had (7 chargers, none worked without a warning on screen).