Because this wouldn't be targeted towards a single device/connection. This is for a household of 5+ streaming 4k, running servers, having cloud (yada yada) IoT devices running simultaneously.
It's the hobbyist tier. It's like asking someone "why do you ever need more than one cast iron pan" when they're into cast iron pan collecting.
20 gig is more than a typical enterprise server currently has. No household needs that much bandwidth. How many households even have more than 3 4k devices at all, let alone using them all simultaneously to stream 4k content.
This is aimed at people who pay 500€ for a pan because it has been treated with handsqueezed mink oil. Stupid enthusiasts with too much money. They will find those, no doubt, but it's still stupid.
PS: just think about how much power the modem alone will draw.