I was being sorta facetious there. The real answer, I suspect, is partially that, and partially down to how a lot of the ecchi series double as the shitty faceless SI protagonist series. It's probably as much about imagining themselves in that situation as anything else, for much the same reason as why shitty SI adventure series are so popular.
... to be honest, I'm pretty sure it's not that. Mostly because I've frankly never actually seen people react to series that way, despite folks saying it's some kind of weirdly common thing. Seems to be more something people that don't enjoy the works and can't particularly comprehend the appeal say to denigrate the people enjoying it, really. Shitty SI adventure series tend to be common because people enjoy
writing them more than reading them, just as one particular counterpoint, in my experience, and silent/low impact protagonists in general allow the writers to focus on other characters.
If you're looking for actual differences, there's a fair number of them. Ecchi series actually end up multivolume fairly often, whereas similarly structured porn (i.e. drawn or animated hentai) does only very,
very,
very rarely. Even larger porn works (OVA series, the tanks and whatnot) often don't even go the entire bit of a single volume/episode (especially with drawn stuff) focusing on the same story/world. Even if it's shallow, there's almost always significantly more character and plot investment with the non-straight up porn stuff, and that actually does draw people (see comparative live action with its 1001 interchangeable daytime dramas that also include plenty of endless teasing and softcore porn). There's the occasional bit of exception in the world of porn (which often makes for some of the
best porn, at that), but it's not even
remotely common.
Due to the fact that it's
not straight up porn, the general structure and writing flow are also generally pretty different -- you have more humor, more slice of life, more etc. The supporting bits of lascivious works get more screen time they don't get in porn because in porn 50+% of the work is dedicated to people actually fucking. Even if 50%+ of a lewd non-porn work is dedicated to fanservice, it
still is almost always significantly less metaphorical real estate consuming than screwing. The focus is pulled back a bit, which allows other things to get in the picture, often stuff people
also enjoy. Sure, it's still often what amounts to softcore porn, but there's plenty of reasons that people with cheerful access to vast swaths of explicit stuff still consume the less explicit stuff.
'Nother big part of it is that most of the particularly popular ecchi series are... actually not that bad. They're not "terrible". Usually not the most amazing thing to grace the earth, no, but people that are fond of these works do actually have something of a degree of ability to discern quality. The works tend to be pretty shallow, but a lot of time that's not actually a necessarily
bad thing. Sometimes people just want something fairly straightforward and/or focused -- see basically every action movie ever, ferex. But with the series actually mentioned... I spoke on musume a bit back, and as I noted I'd call it pretty damn competently done. DxD actually has fairly interesting worldbuilding and occasionally pretty neat bits of writing/character interaction/etc. -- there's a
reason it's been generating a surprising amount of damn solid fanfiction, and it's not just the busty exhibitionist redhead and assorted other half to un-clothed characters.
Other things have other draws -- I'd fight the person that said Sora no Otoshimono was shit, ferex, because it's up there on my list of goddamn amazing manga despite (because, to an extent) having large tracts of its land dedicated to large tracts of land. Shinmai Maou no Keiyakusha, one of the recent-ish ones that is
particularly egregious on the lewd front, I'd still call pretty enjoyable mostly for the art (and not even the explicitly fanservice part of it -- the succubus makes some amazing faces) and character design, even if much of the rest of it is kinda' wonky. Nana to Kaoru is the fluffiest borderline-explicit BDSM fiction I think I've ever seen. People around here seem to enjoy GATE, and that's fanservicey as
goddamn, and often in ways twice as squicky as stuff like freaking prisma illya, which at least doesn't go out of its way to marry its loli-baiting to bloody mass murder. There's just a list that keeps on going. You have to be pretty much explicitly blinding yourself to denigrate the fact that a fair amount of these "terrible ecchi" (i.e. fanservice heavy) series actually have a pretty damn large bit of merit behind them -- oftentimes in fairly specific areas, but still.
I could probably keep going, but really, what it breaks down to, as ree intimated, is that people can actually enjoy differently focused stuff for different reasons, and that enjoyment can actually be pretty legitimate. They don't usually have one or two specific interests that consume the vast amount of their decision making process, and can appreciate different facets of a work to different degrees.
... also, totally ended up writing more than I expected to, there. Oh well.
