Well...
1) If singles just keep being dished out, fans won't have time to "recharge their wallets", probably causing a "ehh, i'll get the next one" mentality (?) and end up wasting good songs with not so good sales
2) Tsunku's a genius, but we can't expect him to keep dishing out songs without giving each a good amount of time/thought
3) Time: Girls are pretty busy I assume because I guess most of them are in school, then when they're not they probably have training/lessons/practice for concerts and then there's handshake events and tv appearances, radio shows, etc
Practicing new songs/choreography also takes a bit of time.
Not sure how long it takes copyrighting their materials take, and there's probably a whole bunch of business decisions, meetings, budgets, outfits, etc. that needs to be taken into account first, as well as trying to plan promotions for a release.
4) People don't get bored of songs THAT fast, do we?
But yeah, as you've all confirmed, the whole process into making a PV/MV isn't really that long, as I would expect, especially at how budget H!P is (even with our recent bump in effects and stuff
) But there's a whole business behind it...though, again, no expect or anything...