If you imagine an average game, it can go something like this: First turn Jynx mirror. You press lk. So do they. 50/50. They get the sleep. You can stay in expecting them to blizzard, you can switch to chansey, etc. Let's say you switch to chansey. They fish for freeze. You press sing. You sleep them. Now what? You have options, but let's say you press thunderwave so you para whatever comes in. They switch in chansey and the chansey is paralyzed. Let's say you think that they'll press ice beam instead of thunderwave to try to fish for freeze, so you throw in snorlax to catch the ice beam, so you now can put pressure on with lax. They throw their lax in to take the body slam. Maybe you press body slam again here, maybe reflect. But regardless, you eventually both end up pressing reflect. As you can see here... nothing interesting is happening. A lot of
stalemate-y moves. There isn't a lot of room for taking a lot of leverage here. You'll have to gradually fight for it unless you get lucky (such as the jynx that gets the sleep first freezing whatever pokemon that comes in afterward. That's two pokes down in a couple of turns). If you try to fight a mirror, throwing a right punch just causes the reflection to throw the same punch. You're not getting very far fighting your reflection. To be clear, the better player is going to make marginally better plays here which, while marginal, will lead to big rewards (the win). But that leverage is gained MARGINALLY, if the other player isn't bad. Stalemate moves gradually become winning or losing moves.
https://replay.pokemonshowdown.com/gen1ou-2250410097-xvg3ghb391dlafzxa8n49nahry3iygdpw
I've posted this elsewhere, but if you watch this game, it is a fight to the death from the very beginning, though the beginning is a little boring. Our team structures are so different that we can't throw the same punch. Pressure is on from go. This means you are making game winning moves or game losing moves (as opposed to stalemate moves) the majority of the game, and the player who plays the most intelligently is going to win (it goes without say that this goes out the window when rng rears its ugly head.) But hopefully this is slightly clearer in conveying what I mean.