It's worse than that. It's due to some weakening in the checks and balances system of US federal regulations, as well as a hole in the system.

Historically, laws were written by the legislative branch (congress), enforced by the executive branch (the office of the president), and overseen by the judicial branch (federal courts). When oversight works, all three have to agree for an enforcement action to take place.

In somewhat recent history, congress has been passing regulations much less, instead passing authorizations for the executive branch to write their own regulations. On top of this, the judicial branch had, until last year, allowed executive organizations to use their own judges, without juries, and follow their own interpretation of laws and regulations, when it conflicted with a literal interpretation. See https://en.wikipedia.org/wiki/SEC_v._Jarkesy and https://en.wikipedia.org/wiki/Loper_Bright_Enterprises_v._Ra... for the recent rulings overturning those precedents.

The effects of those last two cases are still working their way through the relevant bureaucracies, so until then, the FAA will keep with their current interpretation of gaining experience as a form of commercial income (https://www.faa.gov/media/15611) and having a reputation as form of advertising (https://www.faa.gov/documentLibrary/media/Advisory_Circular/...).

Even then, there's a hole in the system that especially impacts overly broad definitions of commercial activity, which as far as I'm aware, no other executive organization has taken anywhere close to the extreme of the FAA.

That hole is that even with the precedent of the recent supreme court rulings, they only take effect when something goes to the courts. With real commercial activity, this happens regularly and often preemptively, because the costs and resources needed to take something to court are easily recovered by the extra income or savings of a ruling that follows proper checks and balances.

On the other hand, there is no incentive to spend the money and resources needed to regain rights for non-commercial activities, like carpooling (or airplanepooling) where no one is earning anything.