It's because min-maxing feels competitive. D&D is a game. You roleplay and roll dice, you don't compete for money or prizes. Min-maxing often leads to you being much too overpowered which makes the members of your team feel useless and uninterested and make the DM sick of you ruining every encounter they had planned. Throwing 3 red dragons into a level 5 encounter to counter a single player trying to be special and OP is not fun if you're not trying to run a Tomb of Annihilation game.

D&D is so open and full of possibilities. You can achieve anything if you do it right and roleplay well. But people who min-max are basically saying "No, I win because I said so.". It feels like playing with a child who makes up rules as they go to make themselves win. It completely ruins the balance of the game.

Now, there are different levels of min-maxing. There is literal min-maxing : you are great at some things, but suck at a lot of other things. I think this is a great way of playing. For example, a Wizard in a game I'm in has 20 Intelligence, 17 Wisdom and 16 Charisma. He has a ton of utility spells and can do basically whatever he wants with them. But his other stats are thus : 6 Strength, 9 Dexterity and 8 Constitution. He's the kind of person to be picked last in dodgeball over a paralyzed kid. He has never, in the history of our campaign, succeeded any physical roll. And it's the funniest shit. I think this type of playing is great : he used the rules as intended and worked with the DM to homebrew some things and balance his stats to make him shine as our utility caster, but letting our other party members be the focus of attention as damage dealers and physical problem solvers.

But then, there's the busted shit. The stuff that's not fun. I'll use the Coffeelock as an example. This build is min-maxed so that you have infinite spell slots. Do you understand how that breaks the balance of the game? People are limited to single-digit numbers of spells for most of their levels for a reason : spells are powerful and versatile. The Coffeelock is a multiclass of Sorcerer and Warlock, Pact of the Tome. It goes like this : Warlock spell slots reset on a short rest, but Sorcery points reset on a Long rest. Convert all of your Warlock spells to sorcery points, take a short rest, repeat. You keep gaining sorcery points. Use the Moon Aspect invocation on the Pact of the Tome Warlock pact boon to negate your need for sleeping and boom, you no longer have to take a short rest, so you can get infinite spells. But here's the thing : spells are literally anything. You can fix any situation with the proper spell, which you now have an unlimited amount of. How is this fun for the DM?

The Dungeon Master is also there to have fun. Ruining all of his plans and consistantly forcing him to find bullshit ways to counter your overpowered character is not. Basically, min-maxing is being invited to a pillow fight and bringing a glock.