Rec classes are a bigger money maker though. For meets adding an additional level, especially if it could be combined with another level's session that would be smaller anyway, would be a moneymaker but that's less from the gym side. L2/3 preteam or competitive teams must have less overheads than the more specialized and longer training hours of higher levels, but I'm not sure that "competitive" L3 is all that more profitable than L3 "preteam", or than L4. When I was a kid, competing L4 was pretty much like competing L3 now. But compulsories have also gotten much more competitive as a whole. There were way less 9s, and I don't think this is completely scoring inflation. When I watch a compulsory meet now, I think the actual quality of gymnastics is better and the expectations are higher than they used to be.
I suspect for most gyms it's more of a way to involve people early, and they also feel it gives them an advantage in L4 and 5 to have had prior competitive experience now. There may be some gain to having more parents in the parents association, things like that, but there are also costs to arranging competition, for example whoever is coaching at the meets has to be a pro member and get a background check and safety cert...for many gyms this is going to require an additional person with these credentials because of session overlaps at meets and of course it should be the person who is working with the kids most of the time. With training hours you're paying less per hour the more hours the kids go. The gym needs to purchase materials, learn the routines, etc.
I see it as more of a trend, much like L4 competition. Now we are moving towards USAG pushing lower level competition, which is a moneymaker for USAG, a way to grow the sport. Individual gyms I suspect mostly are indifferent between the gain to have their preteam compete in sanctioned meets or not. Unsanctioned meets would come at less cost.
We don't really have L3 competition in my state much yet. There are unsanctioned, informal competitions between preteams, and some people have enrolled their L3s in the lowest level of prep op and just had them compete the L3 routines there.