There's a ton of software that assumes the main branch is labeled master.
That software is broken if it doesn't allow you to configure this (just as git does).
You're right. But there's a lot of broken software out there that just trudges on with momentum.
That said, I'm pretty sure you can just make a main branch and delete master in your personal repos. I don't think git itself needs master, does it?
There's a lot of 'broken' software that assumed any Windows version number with a 9 in it was made in the 90s. That doesn't mean that we can (nor should) throw out that software.