Not a technical thing I believe.
As for why, it was always explained to me that the majority of internet use, for residential customers at least, was a small ‘request’ for data and what was requested being a larger download ....
Look at the speeds Virgin offer, small uploads again.
I guess if you want symmetrical you are still looking at ‘niche’ stuff, private wires, etc = more cost.
Then again, the likes of Gigaclear offer symmetrical services, I wonder what their take up is like ?