Only a few buffs go cross team, like barriers, pains, konan and playboy. You will notice all of those have the key world "field" in their skill description.

If there is no "field" keyword, then it is by default team only.

That's hardly the reason, 3 of the same pain, for example, doesn't stack. In fact, the only thing that the same "buff" stack is shield, no stat boost of the same sort stack, even if they are field wide.
If that weren't the case, lightning main with generator talent and lightning boost along with shurado, chikusodo and tendo x 3 would be devastating...

Fact of the matter is, global stuff are all at a premium. So your cheap, free farm-able stuff simply aren't going to have awesome skills, active or passive. The closest thing you gonna get in form of an exception would be active barriers, the only totally free global effect (well, front line sweep sort do that to, but considering it usually have a limit of 3, it ain't really..)