I think it's easier for billing systems to have 1 subscription for 1 customer. The reason behind this is mainly because you can apply several prices to the same feature belonging to two different subscriptions. Stripe does not work on it because they might consider this as a edge case.

When you look at usage-based billing, even if a billed feature is linked to consumption, it needs to be related to a subscription given the fact that this feature can be priced differently for several plans. Every company has a singular pricing model in mind and it's hard to have an all-in-one model fitting all the needs. Even when we built internally the whole system for a fintech back in 2016, we had some edgecases creating conflict inside the company.