There's a reason for it, though I believe it will be reconsidered in the future.
WFA has an internal cache DB and a configuration DB.
One of the phases during the upgrade (The restoration) takes the DBs and upgrades them to match any required changes
made to the certified content on the new release. This includes complete rebuild of the DB schema.
While probably possible to do in an in-place upgrade, its nicer and safer to do a clean installation and freshly deploy the DB for restore ( & upgrade) and have that pre-upgrade DB in hand for revert, should a need arise (We cannot be held accountable to all non-certified components, and one may break if it was created in a non-conformant way).
The only difference between the proposed scenario is the uninstall - And surely that's not a big thing to do... 🙂