I always assumed it was because ROTS was never released on VHS.
ROTS was released on VHS in the UK, Far East, some European countries, and maybe Aus mate:-
https://www.amazon.co.uk/Star-Wars-Episode-III-Revenge/dp/B00097E6EK
&
https://originaltrilogy.com/topic/Star-Wars-Episode-III-Revenge-of-the-Sith-VHS/id/54562
They are quite rare, and fetch a pretty penny when one goes up for sale.