It is a variety of issues, all rolled together.
1 - fire codes. The resort has an occupancy limit based on fire codes. While the individual units may or may not be rated for the same limit as their intended occupancy, the fire codes might not allow for 1 or 2 extra per room.
2 - septic issues. If the resort has a septic system, that is rated for a specific number of people. Again, while the resort may have room to allow a couple of extra occupants, probably not enough to increase occupancy per room. An infant still makes use of the septic system, just like any other child.
3- economic issues. While hotels charge per person, timeshares charge for the room. While many hotels won't charge for an extra infant, many also don't charge extra for children of ANY age, or under certain age, such as 18 or 12.
4 - wear and tear of the unit. More occupants means more wear and tear, which results in greater maintenance issues for the owners.
5 - Sleeping accomodations Occupancy is set by the number of beds through the exchange companies, and is based on the available beds, and often couches. Sometimes a unit will have an extra couch, making it possible for an extra person to have sleaping space (sometime 2 couches even),
Also consider that everyone who reserves a room is made aware of the occupancy - owners when they bought the unit, or renters/exchangers when the reservation was made. This has impact on not just your reservation, but on eveyone near you.
Consider the situation posed by the OP - a studio unit intended for 2 occupants. Are the other units near it all studios? Not only do those others expect occupancy to be enforced equally, but they may have reserved based on the fact that these are sleep 2 units, rather than sleep 4. While it is entirely poassible a single parent might travel with an infant, it is not very likely, and those neighbors will not be expecting to hear an infant crying through the night.
It is fine to call a resort and see if they have a policy allowing an extra infant, but most don't, so don't expect it. Those hotels that allow the infant would most likely also allow another adult, for a fee. The fee is not the issue, the occupancy of the unit is.