I have watched a LOT of anime, and it is just a trend I've noticed. It is the ideal protagonist type for a shounen. Not saying they all are, of course.
But I don't understand, ISTP doesn't fit any of the shonen stereotypes. Loving your friends, happy go lucky, extroverted, loving to eat, idealistic, big dreamer, ambitious, unrealistic dreamer, impulsive, imaginative, airhead, innocent, naive, these all fit the ENFP shonen stereotype Goku laid out in the 80s. Berserk isn't a shonen btw, its a seinen. Not saying you said that but just clarifying, I'm just very confused by this statement is all.
Here's a great video on the topic, maybe Kenshiro was ISTP? But ever since Goku, most shonen MCs have been ENFP in people's opinion. People get confused now and mixup ENFP vs ESFP today but in my opinion and others there ENFPs for sure. I talk about it here in my ENFP Anime characters post too:
Supereyepatch 80s Shonen Jump video:
What Shonen Jump Was Like 30 Years Ago - YouTube
My ENFP anime characters list I recently made:
ENFP Anime Characters : ENFP
Funny thing is Vinland saga says on the wiki its a shonen and a seinen both, which do u classify it as. It has a more mature seinen vibe from what I see like Berserk. But shows like One Piece, Naruto, Hunter x Hunter, Dragon Ball are all shonen classics for sure, along with many others.