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Because Game Freak don't give their games away for free and Nintendo won't share NSO revenue. There's also the fact they won't exclude regions from game releases and won't market Pokémon at 18's, both of which would need to happen in some fashion for Gens 1-4 to go up on NSO, assuming Nintendo paid them a live-service fee for allowing the use of proper Pokémon games on NSO of course. Learn how NSO works, tells you all you need to know. Or you could have done what I did, be born in time to collect every game that was worth actually buying, when they were on general sale.
