Green card is for people who want to immigrate into US; US isn't interested in granting citizenship to people living elsewhere (never was), the stated goal is to diversify US population. Given that, such a requirement makes sense - and is very common in immigration procedures around the world.
The rules to naturalize are separate and more stringent than the rules to maintain the green card. Talking about granting citizenship is a separate issue.
Further, it appears that after a year your green card is by statute no longer valid for reentry into the country. You don't lose the green card, but your reentry has to be special-cased by CBP or a consulate.
If you are absent > 6 months you could end up with your green card status listed as 'abandoned', but even if you come back every 6 months that is no guarantee at all that it will not be listed abandoned.
The golden rule seems to be to be more than 6 months per year in the USA, preferably substantially more than 6 months to avoid ambiguity.