Users feel like, "Hey I paid for a 1Gbps link, I should be able to saturate 1Gbps 24/7".
Providers know that 99% of users don't need 1Gbps 24/7, so to reduce the cost of the service, they sparsely provision.
Caps of some form, either on speed or on bandwidth (or N Mbps speed cap after Y GBs of bandwidth) are the way they keep users within the limits of the network.
Ie, if you're overselling by a large margin than you'll need to be capping a lot of people. Not the same amount as the "large margin", but a significant portion of your users need to not be accessing the pipe at 5pm.
This is why I think they should just get rid of caps, and instead give us a meaningful SLA. They won't however, because they have massively oversold their network[2].
[1]: https://news.ycombinator.com/item?id=17306906
[2]: I don't know if they've actually oversold their network, it's just clear that there is data troubles based on user traffic. Personally I think Comcast is purposefully limiting traffic based on marketing strategies. Ie, when Google rolled fiber into Portland, suddenly Comcast's "available speeds" massively spiked to stay competitive with Google. Which indicates that they had a lot more than they were selling.