So arbitrarily, I’m not allowed to use fire damage on the trolls until some npc tells me that trolls are weak to fire?

You say arbitrarily but it's not arbitrary. It is dependant on the situation. If trolls aren't super common and your characters have never dealt with a troll? It makes zero sense that you would know that they're weak to fire damage. Question. Do you know how to escape a car that's upside down and submerged in water? Because if you don't, there are a lot of things that are going to get you killed due to not being aware of what the issue is. Now, you might have learned it in the past due to some particular event or due to reading it in something or being aware due to work stuff or whatever else. But the point is that it's a danger that not everyone on the earth is familiar with despite the fact that it is a hyper common vehicle and water covering the vast majority of the earth's surface.

Now instead of cars and water being everywhere, it's a specific monster in a specific location you've probably never visited and the internet doesn't exist. Want to explain to me how it's "arbitrary" that your character would know the vulnerabilities of a specific creature that is from an area you're not from? That you've got no crossover with? That your character has no experience with?

Your perspective comes from that of a player that is frustrated but not of someone who is looking at the world as a whole. Your whole comment talks about how angry you get from being prevented to do certain things but none of it reflects anything from how the world would work internally.

You call it asinine but it's way more ridiculous to think that as a lower level character from the middle of nowhere that you'd have intimate adventuring knowledge of a creature that isn't super common in most situations.

If you want to play let’s pretend with dice, that’s fine.

I mean that is literally the game... Fun fact on the definition of metagaming.

Metagame thinking means thinking about the game as a game. It’s like when a character in a movie knows it’s a movie and acts accordingly. For example, a player might say, “The DM wouldn’t throw such a powerful monster at us!” or you might hear, “The read-aloud text spent a lot of time describing that door — let’s search it again!”

For a lot of us this isn't a game first. It's a Roleplaying Game first. The way that you want to play is rejecting a lot of the roleplay aspect of it in favor of mechanical benefit. Phrasing that as "play lets pretend with dice" just feels bizarrely tone deaf considering that is literally the entire core concept of the game.

The thing about your comment here that is frustrating to me as a DM is that it doesn't factor in anyone else. It's all about how your plan was ruined and about how things prevent you from doing various things but there's no consideration or reference to anyone else in the party. How enjoyable do you think it is for other players if someone in the party is consistently saying "I would know the thing" and providing no reasonable explanation for why you'd know the thing?