Some Pokemon are outright shit. Being overleveled makes you look passed that.
EXP is intentionally easy to get so that you can use your favorite designs regardless of their "viability" or when you get them. If it wasn't easy to level up (and consequently overlevel), you'd pick 6 or less Pokemon before the second or third gym and ignore everything else after even if you still have over half the Pokedex undiscovered, because grinding up a new Pokemon would be too much of a chore (precisely why EXP All is forced onto us in newer generations).
Not to mention how you, the player, are heralded as the "chosen one", or the "best trainer ever". Gameplay mechanics are in place to support this. Think about it. When's the last time you've actually seen an opposing Pokemon level up in real time? Even if you somehow fail against the same trainer multiple times, it never happens. Because you, and your Pokemon, are "special".
You being overleveled is intentional. It's to make you seem like a badass compared to these professional gym runners and criminal organizations who all know what they're doing, some of which have gone through many years of practice or intense training to get to where they are, only to be stomped by a 10 year old child who just started training a few days/weeks ago. You are canonically a prodigy, and both the lore and gameplay mechanics reflect that. There are entire schools dedicated to learning about Pokemon but for the player everything comes naturally. If everyone was on your level, it'd shatter the prodigy illusion, break immersion, ruin the already questionable pace of the games, and make casual players mad that they're forced to use Pokemon they don't like just to progress.
This thread is dumb.