802.11G launched in 2003, the year after Apple held its public funeral for the Classic Mac OS. In addition, Apple mostly (not 100% but mostly) of got out of the business of making aftermarket upgrades for its own older machines in around 1997 or so.
This is probably technically possible, but neither Apple nor third parties figured it was worth doing, and may have avoided it (even if there was a business/sales case for it) if the slot wasn't built to handle 54 megabits of throughput, to avoid being sued for selling a capacity that couldn't be achieved.
In addition, most home wireless networking was still using WEP keys, which worked fine on the Classic Mac OS, for several years after that.
In a modern context, I suspect the reason this hasn't been done is because using travel wifi routers or ethernet bridges with USB battery packs is "good enough" even for the handful of people actually using old Mac laptops.
As mentioned, WPA and WPA 2 do work in OS X releases, even on older macs, and USB/pcmcia/PCI 802.11G cards do often work with older machines. It's just a bummer for the iMac/iBook in particular because they don't really have those slots.
Not that you couldn't, you'd either have to reverse-engineer the AirPort slot and then design a fresh wifi card around a newer chipset and write the drivers and authentication tools for it, or design a card that's both a wifi client and pretends to be an AirPort card to the Mac and the appropriate management interface for it.
I think that "the port is slow" is probably not that much of a barrier who want this, you can get on the outer edge of fifteen whole megabits a second out of OS 9 on gigabit ethernet anyway so it's not like we'd be missing out on much.