The limits you've stated are not the standard limits on those airlines, but instead limits that apply only to specific flights on (very) small planes.
Carry-on limits vary wildly across airlines, but it has become a common standard for airlines to allow one "carry-on bag" and one "personal item". These terms are not strictly defined, and the definitions for both size and weight again vary depending on the airline.
Many airlines that do define weight limits for "carry-on bag" and "personal items" set those weight limits at either 7-10kg for the "carry-on bag", and 3-5kg for the "personal item".
On small planes, the carry-on bag will not be allowed in the cabin due to space and/or safety requirements, meaning that the only thing you can take on board is a "personal item", and thus the low allowed weight limit.
On very small planes (Air Rarotonga flies planes with as few as 3 passenger seats!) there can be also be weight/balance requirements that need to be handled - this is why you sometimes hear of passengers being weighted before boarding to get that correct! Even if there was space for a larger/heavier bag, having it move during flight could impact the weight/balance of the plane, which in a worst-case scenario could lead to a crash.