Why aren't motherboards mostly USB-C by now? (2021)

There are multiple decades worth of electronics and accessories that were built with USB-A that are still perfectly usable and up to date. Removing that option would require buying and keeping track of several adapters to use things that were perfectly usable one generation ago, but now are not, or replacing them with USB-C versions, which is costly and unnecessary.

Speaking as someone with IT experience, both of those issues mean the idea is a non-starter from a user perspective. From an IT support perspective, it's a non-starter because nobody wants to deal with users bitching and moaning about having to update to a new standard any sooner than is absolutely necessary.

Thus: this.

Usb a is robust and durable. Usblc is frail and fragile. I've broken more usbc ports than i can count. I've never broken a USB a port.

If its not a space issue then usbc can go fuck itself.