Why do adult Gorn even exist when the young can apparently reproduce immediately?

So we know Gorn capture other species, pit them against the Gorn and each other ala the strongest M&M copypasta, and then send the strongest M&M back to M&M Mars space. La’an was the strongest M&M, for example.

I tried to touch on this in my OP, but the problem with this is that unless Gorn generations are very spread out (unlikely, given the rapid gestation period and rather ad-hoc method of implanting eggs), the odds of a "superior" elder beating out all of it's slightly younger competition remains quite slim. This is a brawl, not a neatly organized bracket, and random chance will invariably play a big role in who wins and who loses. The top Gorn from generation A suddenly finds itself as merely a slightly advantaged individual in a whole new field of competitors the overwhelming likelihood is that one of them will prove the ultimate winner, and then suffer the same fate. If the "strongest M&M" were thrown into a fresh bracket instead of being mailed to the parent company, it's almost certain to be toppled. You wind up with a species whose "true" lifespan and adult form is irrelevant, because every individual dies long before they come anywhere near adulthood.

More importantly from an evolutionary perspective, though, the success or lack thereof of an individual Gorn has almost no effect on their ability to reproduce. All the Gorn need, apparently, is to survive long enough to spit on a viable host. Anything that happens after they do that is irrelevant, and thus won't be selected for evolutionarily. And it strikes me as highly improbable that growing to much greater size and having enormous strength (never mind developing sapience) are unlikely to emerge by pure chance without evolutionary pressures making those traits more likely to be passed on.

The "Strongest M&M" problem is probably mitigated by lower density of baby Gorn in the wild than what we've seen on screen so far. If a brood typically manages to winnow itself down to a single individual before any of them can spit on a host, and the hist typically has enough time to travel somewhere else before the next generation hatches, then you have a situation where the strongest of the babies will generally reproduce and then generally have the chance to continue growing into "true" adulthood. Unfortunately that still doesn't answer my second question of how that adult form evolved at all when it's very existence has no clear benefit to the animal's ability to reproduce.

"I sent you your strongest M&M why are your phasers powering up?"

This explains why they never joined Star Fleet.

Do they? In the wild, the babies burst out of a host and are immediately capable of running around and spitting on things, which become infected and eventually babies burst out, onwards and onwards.

The Gorn practice of having separate breeding spaces is clearly an artificial construct designed (presumably by the Gorn themselves) to make it possible to have a functional civilization of adult beings. In the wild, anywhere that has viable hosts is a viable breeding area, and these creatures could not possibly have evolved this life cycle without viable hosts commonly available to them.

The grandparents effect did occur to me, but I'm not sure what exactly these few Gorn who reach adulthood are doing to make their descendants (who they implanted in a host long before they themselves grew to maturity) more likely to survive. Even assuming these adults are in position to assist their offspring, the kids are quite capable hunters and don't seem to need protection against anything except eachother.

This is Daystrom Institute, so although we both accept and encourage Doylist answers, "it's bad writing" is never a sufficiently substantive response.