Old people can be stubborn, and often it is impossible to control what they do, especially when they are on another continent. The risk is higher than you might think that they choose to stay in the old country longer than 6 months and put themselves at risk of abandoning the green card - a very expensive problem.
But there is insurance against this development (at least for now- but it might not be available much longer), and that is a re-entry permit. For more information, visit:
https://www.uscis.gov/sites/default/files/document/guides/B5en.pdf
- curtis