Bandwidth is only the speed you reach your cap. A cap has nothing to do with your bandwidth, it's only your shitty ISP wanting more money. 'All' is relative, I have 1gig/1gig with no caps.
Sure a cap could be the issue if you have stupid shit like that, but you won't even reach your cap if the experience is so horrible due to lag that you don't want to even use it.
Generally, an ISP doesn't 'buy' more internet on the 1st than the 31st. We oversubscribe everyone, and pay for fixed bandwidth interconnects through the entire month. It would seriously be a pain to turn up and down circuits based on bandwidth usage or be charged via a meter. If we were to add caps, it would purely be for profit as we have to buy the bandwidth regardless on the 1st before you are even near your cap and our interconnects only charge us for speed, not data usage.