I guess it's because of which types happen to dominate MBTI discussion online, i.e. the INxxs:
A lot of introverts wish they were extraverts, at least rather more than vice versa, and a huge proportion of MBTI enthusiasts are INTPs, so their most envied type is naturally going to be ENTP. Then there are all the INFPs who wish they were thinkers because they imagine they'd be less emotionally vulnerable, in addition to wishing they were extraverts. Js rarely wish they were Ps, but INFJs will speak highly of ENTPs just because we're very compatible, evidenced by the fact that it's mutual. INTJs are the only INxxs that find us just as annoying as all the non-INXXs do.
I'm kind of thinking we lucked out. I'm 61, just found out about types 18 months ago. Until then I'd thought everyone thought like I did. Since then I've learned about some of the quirks a few other types have to deal with. Confidence for example. I can be insecure, but it seems no where close to what others describe. And I can snap out of insecurity pretty fast and easy. It's hard for me to imagine what it's like for others.
Being any ambivert is sweet imo. Extrovert is way over rated. The more extrovert the more superficial which is fine for them, but I wouldn't want that. Being able to make lots of connections AND still have Ti is way sweet. Great substitute for actual high intelligence. I've worked with many very intelligent people, but they miss many things. I wouldn't trade brains.
At this point in history, in the US, things are really setup for an ENTP. But, and it's a big but, I found being an ENTP to be difficult to fit into many parts of society. I worked almost exclusively in startups and in that world it was heaven. Doing whatever I wanted, working with similar or enabling people. But outside having to deal with other types, that was years of hard work.
I would say that being a INTP who has dealt with hangups about everyone wanting them to be more extroverted would be pretty cool. Probably true for all types.