I am curious: isn't there such requirement for green card holders in the US to meet to keep their status?
There is. 6 months in a year or so is the maximum you can be out of the US. If you come back a year later, with no special permit (I believe there is a certain kind of permit you have to get, if you will be out for more than 6 months) then you are subject to forfeiture of your PR status. Also, US permanent residents don't have half the rights of Canadian PRs. Canadian PR is entitled to enter Canada, no such right exists for American PR, under certain circumstances your PR card can be tossed away and you can be detained, with no end to it in sight.
But! Very few US PR's stay out of US for long period of time. Some people do that (and I knew at least one), but overall it's extremely unusual for US PR to go out of US and stay somewhere else. Mainly because of economic conditions: if you have a right to work in the US you WILL get a job, and fairly quickly.
Anyone can land a job here in days (weeks at most), IF THEY WANT TO. So, while US has much stricter enforcement of RO laws, the fact is too few are ever in breach. You are not forced to leave the US, so you can have better earning options elsewhere. So, you don't feel like the law is pushing you to act against your existential interests.