Because GPON is asymmetric and that is the cheapest way of deploying fiber optic for residential use.
If you want symmetric, then you need a lot of "active" equipment and that drives the costs up.
If you want symmetric, then you need a lot of "active" equipment and that drives the costs up.
In any case GPON is already in its way out, new deployments use XGS-PON which is 10/10
But either way it's irrelevant to the point that the technology can very well support symmetric speeds and offering symmetric doesn't "need a lot of "active" equipment and that drives the costs up." as the commenter I originally replied to claimed. Also evidenced by XGPON having the exact same amount of active equipment.
On the other hand, more than 300Mbps upload is quite ok even with heavy video conferencing and what not.