Over-subscription is literally how the entire internet works. Most devices spend 21 hours doing a whole lot of nothing and 3 hours either doing really quick bursty things like spending 3 seconds loading a page followed by 3 minutes interacting with it or relatively low speed things like streaming 50Mbps. Having a higher speed just means that when you want something to happen it happens quickly and it happens even if you have 12 other devices doing the same thing.
Normal internet is oversubscribed by about 20x and gives most folks 90% of their plan speed at the modem most of the time. Dedicated bandwidth by definition means that you rent enough capacity for them to serve 1Gbps every second of every day even though you will use almost none of it. For reference 1Gbps for a month is about 327 TB of data. Most people use between 0.1-3TB over the course of a month.
Dedicated connections are a lot more expensive to provide and a lot more expensive to contract for. That 1Gbps connection right now costs about $1000 per month. Your requirement would require ISP to sell only much lower connection speeds for at much higher prices. It would in fact actually break the internet as we know it. It's not exactly shocking to imagine that buying 100–1000 times what you need is expensive. A better standard would be to enforce 90% of plan speed 90% of the time measured at the modem with a week to correct if less than acceptable. Some european company actually makes an app to enforce their particular standard and takes the guess work out of measurement. I like the idea.
Also its impossible to guarantee that customers will in fact even reach those speeds over wifi as its a function of the customers actual space, materials used to build the home, what's in the wall, network hardware, AND wireless clients. You only get really fast connectivity over 5/6Gh which is short range (100-200ft), only with quite modern equipment on both sides.
This means that your 2015 $200 modem/router combo with 2018 clients is probably giving you 300Mbps in your living room and 50Mbps upstairs even if the modem itself is getting 1 Gbps. This is just how wifi is. Your ISP isn't going to be responsible for installing a $1000 worth of hardware so you can get plan speed upstairs on your $20 a month service. There are contractors who WILL do that for you for a hefty price. You'll be paying for the $1000 worth of hardware and a professionals time.