Short answer: the AIM is giving you more details on how to comply with the regulations, it isn't just repeating what the regulations already say.
For context, the AIM itself says in its opening section (emphasis mine):
This publication, while not regulatory, provides information which
reflects examples of operating techniques and procedures which may be
requirements in other federal publications or regulations. It is made
available solely to assist pilots in executing their responsibilities
required by other publications.
Going back to 91.183(c), it says that a pilot must report:
Any other information relating to the safety of flight.
A pilot might wonder exactly what information the FAA thinks is related to safety of flight, and how many reports ATC really needs to receive. The AIM provides guidance by listing specific reporting items. It's like the FAA is saying "report these things and you'll (probably) be in compliance with 91.183(c)".
Another example is 14 CFR 91.103, which says:
Each pilot in command shall, before beginning a flight, become
familiar with all available information concerning that flight
"All available information" is very broad and the AIM provides more specific guidance in a couple of places including 5-1-3(b), which essentially says "check NOTAMs before your flight". I assume most pilots would agree that checking NOTAMs is an obvious and essential preflight step, but 91.103 doesn't mention them at all.
In other words, these are examples of the AIM giving more details on how to comply with a regulation; it isn't just repeating the regulation.
You might still ask, if these things are so important then why aren't they in the regulations in the first place? One very general answer is that changing regulations is extremely slow and difficult, whereas the FAA can publish a new version of the AIM whenever it likes. Procedures and technology change much more quickly than regulations can, and there's far too much technical detail in aviation for it to be practical to have it all in the regulations themselves.