In any western Soaps or the likes, the characters hook up almost immediately and then work their way through the rest of the cast and new additions.

Gossip girl is a really good example, they have literally worked their way through everyone. It makes NY look like Kansas.

In Japanese stories it takes forever for a kiss. I mean most characters on a soap has more partners than there are kiss scenes in a manga.

It's pretty hilarious sometimes to see the differences.

Well, it's not that much like that in Josei, but yeah. This is hardly a "mistake" tough, remember that manga comes from a culture where kissing in public was seen as indecent until external influence made its way into it, and even now it is't as common as in the occidental world.
To top that off, we are talking mostly "Shojo" or "Shonen", aimed at kids and, lets be honest, misfits mostly. The majority of the readers holds the romantic and classic idealism of a poetic love, because they might not had experienced "modern" love at all.

It's simple - stable relationships just aren't as interesting to watch/ read. There's no drama. So the easiest way to create that drama would be to either delay the couple from being together, or to have them hook up and break up quickly.

Ofcourse, you also have to remember that sex sells. Are there any teen dramas on TV that don't feature that kind of fanservice? A show that doesn't have deaths or sex is a show that the media doesn't care to talk about. You can't have much fanservice if the main couple haven't even kissed yet.