Winding your own transformer is really not that big of a deal. Especially if you work in R&D as a power electronics engineer. No one waits for the custom transformer to be delivered to your bench when you were designing a new power supply.

For high power stuff though, you need some equipment to do that since copper will hard to bend neatly.

I didn't realize this was a think until I heard of people doing it in a a CRT forum.

I suspect it will be getting more popular as flyback transformers get harder and harder to find.