Charizard has a tool that allows him to bypass most projectiles from the other side of the stage and allows him to punish from there while providing the best recovery of any heavy. DK and Bowser do not.
Charizard has an super armored Up B that allows him to kill out of shield while providing an actual vertical recovery move. DK and Bowser do not.
Charizard has an effective option to gimp characters with linear vertical recoveries without leaving the stage while also providing a safe jump in against any character. DK and Bowser do not (unless you are REALLY lucky with Bowser, I guess.)
Charizard has a disjointed super armor move that gives him an option to safely land that can land up to 30% if it hits right. DK and Bowser do not.
And that's just the specials. I'm not even going to get into the nuances of his stats, normals, grabs and smashes like the fastest jab of the three you've mentioned, the best dash attack of the three, the best up smash, the fastest dash, three jumps, etc.
TDK
You're ignoring all of the things that make Charizard unique. All of his nuances, his quirks.
It's like me asking why you main Corrin when Cloud is the better long range swordsman with poor recovery (which Cloud can improve btw, Corrin is stuck with his).
It's ignoring almost everything and shows I don't know the nuances between the two, right?
It's focusing on only certain elements and to an absurd degree.
I'm not going to sit here and say Charizard is the best character ever. He's not and he has definite flaws.
But his flaws aren't killing.
And his flaws aren't being a worse version of Bowser or DK.