I'd like to point out that one of the bad OELVNs you've read (Gogo JP) was actually made by a Japanese developer, Overdrive...
You can't generalize and say that all OELVNs are bad after having read just four (three if you don't count the Japanese one). I'm not saying OELVNs are good, in fact I'll be the first to say that most of them are bad. But, there are some hidden gems to be found, and the quality of their stories and art makes me think that if they had the same financial backing and experienced production teams that Japanese developers benefit from, they would be of the same quality as great Japanese VNs.
But to answer your question: I'll admit that I haven't read that many Japanese VNs. The only really good ones I've finished have been Aiyoku no Eustia (https://vndb.org/v3770) and Tsukihime. I tried to read Shiei no Sona-nyl (https://vndb.org/v4936) but couldn't finish it, the Japanese was too hard. That said, I've also read some fandisks, as well as a lot of shorter VNs I didn't like. Off the top of my head, stuff like Juukishi Cutie or Maro no Kanja wa Gatenkei... I've read many more translated VNs, though.
From those experiences, I can definitely say that there's a big sense of accomplishment and mystique that comes with reading a good VN in the original Japanese. I love Japanese culture (like you, I suppose), and I'd much rather read a great VN in Japanese than in English.
None of that changes the fact that a lot of Japanese VNs tend to be very bad, whether it be in terms of story, art, music, gameplay, or whatever. It's not a knock on Japanese VNs, it's just a fact that in every market and artistic medium there tends to be a surfeit of poor-quality content and a small amount of very good content that distinguishes itself from the rest. And, you have to recognize that that problem is exacerbated by the unique characteristics of the Japanese VN market. It caters to an audience that often cares more about h-scenes and moe than innovative plot, gameplay, or art, meaning it has no incentive to challenge itself and produce quality content. It's very corporate and formulaic--there are many companies just like Winged Cloud in Japan that pump out nukige after nukige and manage to break even by targeting the lowest common denominator. Like Toranth pointed out, there are also huge numbers of doujin developers that flood the market with works of essentially the same quality as OELVNs.
I'm sure you know all that, because it sounds like you read a lot of untranslated VNs. But it's just undeniable that the very nature of the Japanese market means a majority of Japanese VNs will be bad.