Oh, really? Azure wants to have a word with you ... Yes, currently they don't enable this for subscriptions with commitment plans or with pay-as-you-go pricing, but it is not because it is not possible or feasible, as you argue - the technical infrastructure in the form of spending limits, spending budgets and quotas is there [1-3] and is available for select plans [4].
Translation: "We have spending limits, but only for accounts where stopping the spending is important to us. For accounts where the spending is important to you, there cannot be a limit configured. Even though we wrote the code. In fact, we had to add extra code so that we could selectively remove this option."