Shoutwiki is a wiki farm that offers some features Fandom and Gamepedia don't provide, but also the other way round. And you say "should merge with" as if that's something that can just be done. If Shoutwiki's finances are at a state where the owners are looking for outside investors, maybe. But it's not like Fandom just picked Gamepedia out in the shop.
You seem to have an obsession with things merging - without understanding how mergers work.
Shout Wiki is a completely different entity from Fandom, which is fine. There can be multiple wiki farms that serve different needs and purposes, or even if they aren't that different, it wouldn't really matter.
Fandom isn't a flagship wiki farm for the entirety of the internet, and I don't think it's trying to be.
It's good to have these two platforms separate instead of combining them - there will be a load of conflicts. ShoutWiki probably has some features that you prefer on Fandom, but that doesn't mean the same for everyone else. I never gave ShoutWiki a second look - not just because there was already a pre-existing (abandoned) Fandom wiki for the topic I wanted, I really don't like the design of the ShoutWiki, the editor (I know it's also used by Wikipedia - hate it) and it feels more complicated to use than Fandom. However, if people prefer those features, then Shoutwiki will work for them.
The impression I got is that Miraheze and ShoutWiki more or less stick to vanilla MediaWiki plus some public extensions. Fandom has custom extensions and styling so yes things will still be different. However, with the new direction of aligning more with vanilla MediaWiki, Fandom/Gamepedia wikis will probably look closer to Miraheze and ShoutWiki than they previously have. But that is just a guess on my part.
Personally, I like how FANDOM keeps the main article content mostly on the left and has a right rail rather than Gamepedia, Wikipedia, and vanilla MediaWiki with their left rail. If I came from a right to left culture, I might think otherwise, but that's how I see it. It bothers me enough that it discourages me from contributing to Gamepedia, so if FANDOM switches to the vanilla style, my contributions will probably drop.
Perhaps but the purposes of the rails are different. The rails on the other farms are for navigational purposes; what Fandom puts at the top. Gamepedia still has ads on the right despite also having the left rail.
I think, also, with MH and SW being "vanilla Mediawiki", they look less welcoming than Fandom. Fandom has a more modern design, and gives admins the option of customising the wikis to look even better.