Data caps aren't irrational, the infrastructure the ISPs build around the backbone has a maximum capacity of data throughput which needs to be shared between all users, data caps are used to make sure that some users don't detrimentally slow down the network for others by constantly uploading and downloading large files.
Think of data throughput not unlike gas or electricity, there's a per unit cost to provision it and most ISPs don't charge per unit but rather sell packages where the amount is flexible between certain amounts, normally because some people don't use much bandwidth and so it's shared between a number of users, essentially low users subsidize the high users.
Getting more data is simply, pay more money, most ISPs have tiered packages to offer high data users a better package with more data transfer, you can even rent leased lines which reserve that entire chunk of bandwidth for you so you can download 24/7, they're just insanely expensive (our work line is about £800pcm for 50mbit fibre uncapped)
Games are getting bigger for sure but data rates and bandwidth limits are going up, I grabbed GTAV on my 152mbit line in about 2 hours and it's 60gb, I don't have specific data caps although I do get throttled temporarily if I download a huge amount of data at peak times which is a very reasonable deal.
Some UK ISPs did do this and some may still do it. One of the major differences is that phone networks data usage is tiny compared to the data usage of the internet, voice calls require a tiny amount of bandwidth, the amount of data users can put through the ISP when something like GTAV launches and everyone hammers digital distribution is insane.
The main issue is that users go through peak behaviour, broadband was originally designed to be somewhat burst like in nature, peak usage for a small amount of time when needed, the networks can't handle everyone always downloading max speed. You can give 10,000 users all a $0.05 per Gb charge but if they all decide to hammer the internet at the same time say during some netflix release then your network needs to be big enough to supply all that data at peak, this is why caps are used instead it forces people to moderate out their usage across periods of time. Some UK ISPs had/have on and off peak caps where offpeak had a larger cap because they preferred you using data overnight when usage was naturally low.
Basically what I'm trying to say is that the network has a maximum throughput at any one time and selling on demand per Gb is risky because patterns in your userbase can hammer the network all at once bringing it to a crawl, it's simply not possible to provision high speed internet so that if all your users are downloading at once the bandwidth doesn't run out, unless every user has a leased line and reserved bandwidth and is paying you $1k a month each.