If AVIF was not patent encumbered, AOMedia would not need to have a Patent License to allow open source use.
A majority of the most recent standards are effectively cabal esque private groups of Corporations that hold patents that on the underlying technology and then license the patents among each other as part of the standards org and throw a license bone towards open source. That can all be undone by the patent holders at their whim.
There’s no need to create a standard format that’s patent encumbered especially if they don’t ever intend to monetize that paten,t. It’s all about maintaining control of intellectual property and especially who was allowed and when they are allowed to profit from the standards.