Looking entirely qualitatively (quantitive numbers would be quite confidential) some people store up their Avios for that prized long haul trip in First without any status. BA does a lot of business selling Avios to people like Tesco who then give them to such people. Restricting First redemptions to only status holders would damage that Avios sale business.

Avios is much more than a frequent traveller kickback, it's a whole private currency with it's own multi-actor economy.