Why? What you foreigners call 'Hentai' is hentai itself.
It is ridiculously, vacuously true. You know, it is this fact that 'Hentai' is nothing else than hentai which has gotten some Japanese who imagine thir own country culturally youthful and flawless, no inferior than the western. That's the point of this discussion and nothing else.
Gooooogle hits 2000,000 Hentai.
I don't know good English word to... Didn't learn at school cursing vocabulary...
In everyday Japanese, Pantsu means pants now (as a fashion term).
Because the word Zubon is outdated & considered 'dasai', we can't but say Pantsu.
To say Pantsu to designate Pantsu would be regarded unfashionable, or, as if a girl was avoiding the too much pretty word, 'Pantii'.
I don't use the word Ecchi without verbalizing it. Ecchi-suru, i.e., sleep with some one (generally 2nd person). Say it cool, "Kon'ya, Ecchi-suru?" And she will reply, "Mmmmm, Ecchi nandakara..." Ecchi is a cute joking word, one of the most gentle words in modern Japanese. Almighty word. By no way harmful.
We don't say hentai-suru...
Hentai is the word for a girl, in elementary school, who is upskirted. "...kun, hentaaai! Kimoooi!"
No one who already knows another scientific meaning ever uses this word in this sense. By the way, kimoi (short for kimochi-warui) is the best word to knock down a Japanese (male/female). You can omit the last i. "Kimo!" And s/he will never call you again.
Analysis.
Hentai is old Chinese origin word (Kango) & unconsciously sounds intellectual and schoolish.
Ecchi is English word (...yes. English.) & sounds blondy.
No one knows the etimology of Ecchi. It is from Japanese biggest dictionary (kojien) that Ecchi was an acronym for hentai in a high society girls' highschool, long ago, when there was still a kind of high society in Japan.
No doubt English Hentai will be readapted into Japanese.
Few Japanese read Harry Potter, Sci-Fi, Load of the Ring, American comics, fanfics, &c.
It is like Sakoku again. & westerners appreciate with their culturalized and historical and equality-minded objective eye Ukiyoe (manga).
I am not proud of our manga... Cuz would be nationalidiotic. But it is the BEST art form we have by now.
Sorry, I was forced to state all about the subconsciousness of japanese feels.
Hobby is hobby & don't take it hard!