F9 has to be able to support itself horizontally for effective road transport. Starship and Super Heavy will never be road-transported - they're too large. Any transport that they don't do on their own power (which should be considered the modus operandi...e.g. airplanes take themselves to airports and in for service under their own power) would have to be by sea. And while it'd likely be easier to mount the rocket horizontally, it certainly could be transported vertically as well - even without any cargo, a Maersk Triple E stands higher than a Super Heavy, for example. And SpaceX already will be designing barges specifically for it to land on, as with Falcon 9.
That said, it's a good question as to what structural requirements reentry impose on Super Heavy and Starship. But it's important to remember that during flight, these vehicles are pressure-supported, so it's not a simple question of "Are entry forces greater than 1G?" SpaceX has long since made it a decision to not require pressurization for surface transport (balloon tanks) due to the difficulties and costs such an approach imposes.
Huge VABs are uneconomical for building large ships or containing refineries, and given how cheap Musk wants Starship/Super Heavy to be, one shouldn't expect VABs for them either - especially given that stainless is far more weather tolerant than alumium. And we've already seen SpaceX building their test rockets outdoors, and apparently they think that's going to work just fine. So I just don't see VABs in their future.
But that's just my take.
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