Greencard status remains until either revoked or returned. You cannot apply for something you have.
When discussing immigration there is third (there are actually others but not relevant here)... abandonment per 8 CFR 211.1(a)(2), if LPR remains outside the U.S. for more than one year, the regulations require invalidation of this green card, when this occurs the DHS takes the position that residency also has been abandoned.
But, if we are going to quibble, then there is no such thing as greencard status...its immigrant status and a visa (or other entry permit) is simply what allows one to enter the country with that status.
The DS-117 is an Application for Determining Returning Resident Status and part of the outcome is a consular adjudication of your residency.
If the application is denied, you have been deemed administratively to have abandoned your lawful permanent residency of the US and thus you lose your immigrant status.
The only option at that point becomes a new petition for an alien relative (I-130) to obtain a new immigrant visa.
If the outcome is positive then the applicant is deemed to have only been temporarily absent from the US (and thus residency is not deemed to be abandoned) and an SB-1 Special Immigrant Visa can be granted.
Yes, the OP is right that technically one can turn up at the border and have ones status adjudicated by CBP officials.
But agree that the direct entry route and adjudication at CBP entry while having the advantage of not having to go through consular processing and the long wait times has to be balanced against the risk of an adverse finding at the border which could result in a finding of inadmissibility in the worst case scenario.