Why do asymmetric plans exist in the first place anyway? I assume connections between autonomous systems are symmetric full-duplex, so doesn't these mean ISPs have the same amount of uplink bandwidth to divide between customers as downlink?
a) Most non-fiber connection technologies have limited bandwidth to split between upload and download. If you are offering connections over those, offering a symmetric speed means you are offering a lower max speed (years and years ago, in the single-MBit days, there was a regional German ISP that let you switch your ratio - was very popular with developers and designers at the time, who could reset their connection to be high-upload before sending large files)
b) Only a subset of users cares about high upload speeds (To a degree it's a self-fulfilling prophecy: people are so used to it that most don't even know why they'd want good upstream because they never had the opportunity to do something that would benefit)
c) Some that really want them can be made to pay a lot extra (e.g. for "business class" service, especially if combined with things like static IPs for hosting)
d) some high-bandwidth uses are more likely to cause work for the ISP: people hosting servers (complaints about content etc, attracting attacks), file sharing (rightsholder complaints). Similarly, a customer machine turning into a DDoS traffic source has worse impact if it has high bandwidth
Late 1990s V.90 modems were asymmetric, higher download speeds caused upload speed to deteriorate, and often up traffic would be shunted into a more limited analog channel to keep it from interfering with the down speed. https://en.wikipedia.org/wiki/Modem#Standardized_56k_(V.90/V...
I remember something in the earlier 14400/28800/USRobotics era being asymmetrical too, but can't find a reference.
Where I've lived, ADSL also predates cable modem, and the A there is for Asymmetric.
But, there are also lots of other business and technology reasons as others go into here.
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.