Simultaneous usage and heavy usage is the same thing in practice. Most people watch youtube videos at times of the day when everyone else is watching youtube too. Data caps make users consider whether or not to watch a video via their wireless connection, and hence reduce peak usage.

Telecom companies could work around this by making it cheaper to use data in off hours, but that makes it complicated. Most electricity providers don't have that for consumers either, even though it's actually super expensive to temporarily shut down many forms of electricity generation (and they can't keep them running either, because that would fry the network).