My personal speculation: some other power(s) outside of Ukraine are responsible for using their soft power to handicap the Russian Air Force, whether via hacking or via a secretive-yet-direct physical intervention within Russia.

The "non-news" of Russia Air Force not showing up was not naturally getting coverage in the news, and now DoD figures are making public statements in order to point it out, raise awareness, and poke Putin in the side. Meanwhile, every delay in action from the Russian Air Force weighs heavily in favor of Ukraine.

In this way, major adversaries to Russia that are unable to get directly involved for fear of escalation are still able to place their hands on the scales via more indirect means. Even though there is not official treaty requiring Russian adversaries to intervene, it is often still in their best national defense interests to limit Russia's expansionist tendencies in whatever way they can.

It is all a continuation of major world powers fighting wars by proxy; the dominant pattern of war-fighting after nuclear proliferation became a thing.