I can't find it now of course, but I thought one of the reasons HZD didn't have flying mounts was because the PS4 couldn't handle it... Would be a shame if the PS5 version doesn't get it with HFW because of cross-gen.
It becomes a very different game when you add the ability to just fly everywhere you want so yeah, it's possible that the flying mounts people are envisioning (hop on a Stormbird and fly across the entire map) may just not be something they want. I'm still not convinced that flying mounts are completely absent, those Sunwings or whatever look perfect for being able to cross short distances or easily reach certain heights to avoid having to climb 100% of the time, and that can be done on PS4
I'm in the same boat. I think the devs don't want to offer flying mounts. It would be a largely different game and if the machine denisty is anything like it was on HZD, then it would be rough to address that. I did some attacking from the tops of long necks in HZD and was trivially simple due to machines view radius AI and maneuverability on the head...I just don't think this is something the devs wanted to figure out right now.