Something that's been bugging me.

[Super5]

I'm not sure why the jounin don't take on new students... except that maybe those teams work so well together that Tsunade doesn't want to break them up. Another possibility is that less-experienced Jounin have been taking on the new genin recently.

[Everon]

Eecently we've seen Shikamaru work with other chunnin (granted it was under Sarutobi-Sensei). That's their gentle suggestion that teams often get formed based on needed abilities during a mission.

The eight teams were originally built so that teammates would complement each other's abilities. Along with teamwork experience, it makes sense to use the these configured teams often.