and not Castleoid?

My uncle had Castleoids, he had this special cream and couldn't go on long road trips.

Too much white castle?

Castleoid is too close to Hemorrhoid.

Vaguely think that Metroid was released before Castlevania.

Metroid definitely had the world exploration and map system before Castlevania did.

Why is it called Metroidvania though? Like, what is that supposed to mean? Is it any different than calling it a platform game?

I think cause of the huge interconnected area along with the map, rpg elements, and being able to use powers for short cuts and secret areas.

NOt sure though.

If you've ever played a Metroid game you'll know they're not traditional "level-by-level" linear platformers. I think the term metroidvania was originally coined to describe Castlevania: Symphony of the Night, which took masses of inspiration from Metroid to completely reinvent 2D Castlevania games. From there people just started applying it to lots of games, but I think the most important aspects are the non-linear continuous map and progression controlled by character abilities.
It's a vague and sometimes misused term, a bit like how some people use roguelike to mean anything with random levels and permadeath. Yeah, it's this. Continuous map and progression based on gaining new abilities are key features IMO.

Metroidvania was initially used as a derogatory term to refer to Castlevania: Symphony of the Night, because it copied all its game design from Metroid, moving away from the earlier titles, which were pure platformers. But the term caught on and, just like Doom clones/Rogue likes, it ended up being used to classify a subtype of platformers.

Because of monglets. The same reasons anything with generated levels and rpg style stats are called roguelike.
In Metroid there were paths along your way that were blocked until you found an upgrade or defeated a monster and then went back to unlock the path or get another item.

I guess sidescrolling action adventure, ARPG or platform adventure are considered too vague and some game journalists/writers started using metroidvania at some point, which then caught on with people.

The distinction from other platformers was explained above. But the portmanteau of metroid and castlevania only sort of makes sense since the castlevania series is split into more straightforward action/platformer games and action adventure/arpg games since the 90s.

The important thing is that if someone says "Metroidvania" you immediately think of the features described above and that's much quicker than if you'd have to repeat that description every time. It doesn't really matter anymore if the name makes sense or not, as long as a majority understands what the speaker means. That's what definitions are for, and even if they're stretched a bit sometimes, they can still be helpful. Personally I don't really see the point of being all that strict about them. I mean I get that if you're looking for a very specific type of game, it can be kind of annoying to have all those other titles mixed in that don't fit your precise definition, but going on a crusade and telling everyone how what they think of as [genre/subgenre] is not a real [genre/subgenre] most probably won't change anything about that (especially if you're in the minority with your strict definition, regardless of whether you think it's the right one - that doesn't have any relevance anymore once everyone else is using it differently).

In this case, for example, if someone says the Batman/Arkham games are Metroidvania, they are obviously "wrong", because they're not sidescrolling 2D platformers, and if that's what you were looking for, it is not helpful, but on the other hand, you will also immediately know that progress on a huge map is controlled by upgrading abilities and involves backtracking, and the other person will have conveyed that information to you with only one word. It's also why some games are described as ...-like, ...-lite, or ...like-like, which may seem silly at first, but it's still shorter than repeating the most prominent characteristic of the ... genre, and most gamers will have a pretty good idea what you mean by that.

But since both those franchises are also 3D since the late 90s, metroidvania doesn't necessarily mean 2D unless you're familiar with how it's been used. And it's been used mainly to describe the SotN-like castlevania games so it is kinda strict from the beginning.

If people find it useful then ok, but I think it sounds kinda bad and is unnecessary. If you're into metroid or sotn then you'll probably be into the wonder boy series and faxanadu as well and platform adventure or sidescrolling action adventure works better as an umbrella term for all those games.

Not if the characteristics described above are what you're looking for (open map, gated progress, backtracking etc.); a game that doesn't have them is very unlikely to be described as a Metroidvania(-like) game, but it could very well be a "platform adventure" or "sidescrolling action adventure" because that description fits a lot of different games, e.g. Super Mario, Captain Claw, Heart of Darkness, Abe's Oddysee, Another World, Prince of Persia, etc.; "sidescrolling action adventure" could even be Golden Axe.

If you just go by the name, that is. If your terms would have been established to mean precisely what Metroidvania means now, it would be different, of course. Which is what I meant to say above: "Metroidvania" doesn't have to be strictly related to what Metroid and Castlevania games are (now or back in the days), the name could be totally unrelated to that, which would make it a misnomer or a silly name, but it would still have the particular meaning that people think of when they hear it now, and that's what counts, in the end.