If John dumped Jane at the beginning of the episode, they had to get back together at the end, otherwise the viewer who had to go to her son's wedding that week wouldn't know what was going on. There was no streaming, recaps were few and far between, and not everybody had access to timeshifting, so you couldn't just rely on everybody watching the episode later and catching up.
Sometimes you'd get a two-episode sequence; Jane cheated on John in episode 1 but they got back together in episode 2. Sometimes the season finale would permanently change some detail (making John and Jane go from being engaged to being married). Nevertheless, episodes were still mostly independent.
AFAIK, this changed with timeshifting, DVRs, online catchup services and then streaming. If viewers have the ability to catch up on a show, even when they can't watch it during first broadcast, you can tell a long, complex, book-sized story instead of many independent short-stories that just happen to share a universe.
Personally, I much prefer the newer format, just as I prefer books to short stories.