Yeah I wondered that too for a long time lol. I always figured coax was fine since it handles TV and internet already. But then someone pointed out that all the new smart home stuff and security cameras can really mess with the signal over coax. That kinda made it click that fiber is just more future-proof from the start.
Actually, I read that builders often use coax because it's cheaper and they know how to install it fast. It saves them money upfront compared to running new fiber lines. Still seems dumb to me when fiber is so much better for everything we do online now.
My old router probably thinks coax is cutting edge tech... took me way too long to realize fiber wasn't just for fancy offices. Guess I was future-proofing my ignorance for a while there.