In the companies I've worked for so far since SaaS became a thing you absolutely need to go through procurement for a big enough purchase. You actually need to negotiate the contract each time it expires, which is IMO more burden on the end user than buying a one-off license.
The problem with support contracts, or support requests solved by an upgrade, is that the User needs it now, not after a procurement process.
Doing procurement annually is easier because it can be planned for, budgeted for etc, and happens on a separate thread to the actual support.
Even when they overlap there's enough grace to keep the User happy while waiting on the customer.