Most ISPs advertise "unlimited" as a selling point these days. That doesn't mean they actually are unlimited or that they have no caps. Make sure you check the small print and 99% of the time you will find a cap or, even worse, a reference to some nebulous "Fair Usage Policy" which basically means "we won't tell you what the cap is but we'll kick you off when you exceed it".
BT Broadband and BT Infinity caps
Virgin Media caps (scroll down a bit, there is a table, plus some nebulous FUP-style language).
Sky Broadband caps
And while your statement that none of the ISPs you've been on uses caps is not true, it's definitely the case that many ISPs make this information very hard to find.