My bizarre yet feasible explanation for Kangaskhan having a baby when it hatches: the baby is actually a vestigial twin. All Kangaskhan eggs develop as a twin embryo, one simply overpowers and outgrows the other*. The underdeveloped twin becomes a parasite of the bigger twin. The bigger twin coddles and treats the smaller one like a child due to misplaced maternal bonding and subconscious womb guilt. 'Baby' Kanga never evolves into an 'Adult' Kanga because it technically is an adult. It absorbs all the energy from Mega Evolution to become strong and physically independent.
*Just to make this theory even kookier: the twins are of the opposite sex, and the female twin always overtakes the male in the womb. Hence why all Kangaskhan are female, the males never fully develop, except during Mega Evolution when the vestigial twin grows beyond infant size. They look different due to sexual dimorphism. In Ancient Kanto, this phenomena of female Kanga embryos taking all the resources and choking out the male embryos evolved because female Kanga could mate with other Pokemon in the Monster egg group, and having two sexes wasn't necessary for reproduction.
Fuck your sad ass Cubone theory
Actually eggs in pokemon aren't eggs...mostly for mammals
Noted in the 1996 Dex from the devs, AND subtly referenced in XY, the eggs are pokemon made incubators from grass and dirt. The actual baby mon is placed inside until they're old enough to no longer be fetal
Kanghaskhan just don't do this, since they already have a pouch for development of the baby
Though vestigial twin is a pretty freaky theory
Also note* the 1996 dex noted males exist. But then GSC happened
Also note* Nidorina/Nidoqueen being unable to breed WAS intentional, possibly to mimic how rhinos fail to breed in captivity
Now here's an implied theory based on external events
Hoenn has a subtle nuclear theme
In 2000, the Japanese were concerned over the newly established NUMO project for nuclear power, especially since the older generation still remembers the 2 nukes from WW2. GF seemed to equally be concerned, and the 3rd Gen needed a theme to distinguish it. So they had environmentalism noted across several NPCs, in addition to team Magma LITERALLY planning to blow up a volcano. The effects would be beyond catastrophic
At the same time, there were more direct nuclear references with some mons, mainly the Regis
-Regis all learn explosion at Level one. Since you never encounter them that low, you're forced to remember that explosion. Much like people remembering WW2's devastation...
Other military based moves are also common.
-Destruction Beam (Hyper Beam in Eng) is learned at Level 89. 8/9/1945 (if using MM/DD/YYYY) is the day the bombs were deployed on Nagasaki
-Lockon/Laser Eyes also is learned, relevant for military tech
-Ancient Power's name in Japan is a homophone for Nuclear power
- If the patterns are interpreted as letters, Rock is H, Ice is T, and Steel is O. HTO was a compound used in nukes
-Hoenn's inspiration for the region locale (Kyushu rotated 90 degrees) would have the Regi chambers right near Nagasaki, Miyazaki, and Ooita Prefectures. Areas where bombings occurred
-Incidentally, Regigigas' location is where the memorial in Cape Souya was held
-Accessing with Wailord and Relicanth are other refs. Warlord is the "Fatman" B-2 bomber, with Relicanth as a torpedo shaped weapon "Little Boy"
-Waiting 2 min for some of the Regis references the wait in bomb shelters
-the text on wall in Japan was "In this cave we have lived, and survived" which implies a major incident occurred. In this case, WW2 bombings
-using dig potentially refers to digging into air raid shelters
-The footprint shapes of the Regis is a stretch, but Rock looks like a town/establishment, Ice looks like a tiny bomb dropped, and Steel looks like the explosion radius from it
Gigas looks like an even bigger mushroom cloud,
And the Galar Titans have...nothing
But wait, Gen 8 takes place in Scottland. Doesn't that throw a wrench into this given its Regis?
Surpringly no, since Scottland has had a long history of the denuclearization of nuclear power. Even outright supporting Japan 2017 in not doing nuclear power. Which notably is in the middle of SwSh dev...
So potentially, the Regis and Hoenn in general were nuclear based for inspiration
This is all a touchy subject, so I doubt GF will ever confirm it