The upstream project Mesa disabled those codecs by default. Arch decided to ship those codecs still. Others followed the suggestion by upstream and kept them disabled.
The assumption that patents are already paid by the hardware manufacturer of your graphics card is wrong. I can’t get into specifics, but there are plenty of legal games that are played by hardware vendors to avoid paying those fees.
One example is Apple. If you ever bought a product from them you will have a separate position on your bill stating fees for proprietary codecs of around 5 €. This way you as the customer pay the fee so Apple can provide you with that user experience. You can also read this: Apple-supported H.264 standard gains free license for Internet video use | AppleInsider
As a distribution, we have to ensure we don’t have a way to enable a complete codepath for such codecs to avoid the legal risks. There are a number of ways to approach this, but it’s important that Manjaro itself cannot provide a complete path to access hardware acceleration for these codecs either.
So to answer your question: scroll up as it might have been answered already.