Under normal circumstances, anything that happens in anime would probably never happen in the first place, which is why these special-situations-that's-forseeable-from-a-mile-away are used, and will be used indefinitely. Reason they use such extraodinary but predictable formulas is to start the plot that would otherwise go nowhere without some impetus. For example, your sister/cousin/long-removed childhood friend appears out of nowhere to live near you or in your own house. How many times have we seen that happen in anime, but the odds of that happening in real life are ridiculously low. Or the reverse, anime also have the main character move to a different place to live, whether by own decision or external factors, and ends up with a ideal place full of girls/guys that he/she likes. Or the transfer student that appears in the middle of the year, which usually never happens because it's that rare, except they use that to further the plot along. You'll find anime out there that don't use such formulas, but those are getting rare.