Turnover/churn. If a Q/A site allowed basic questions then the site would be flooded with them since the population of novices greatly exceeds sophomores or experts. Moderation would need to keep up with silo-ing these Qs into the 'novice area' you propose. Experts would need to sift more to get to the interesting, challenging or novel Qs if moderation can't keep up, which might affect the retention of these users, thus diminishing the resource pool available on the site.

To me it's clear that tact and grace is called for, and is currently mostly missing, when directing askers of basic Qs to existing answers, whether on- or off-site but there is such a thing as tragedy of the commons. Some stewardship and curation is required to maintain an acceptable SNR.