Time has the role in it.

If only there were no time compression, Ultimecia never passed the power to Edea, Squall never mentioned about SeeD and Garden to her.... and as a result Edea would never establish the Garden then no FF 8

I think Edea knows Ultimecia will strike again somehow when future comes and Squall turns 17. So she plans to build Garden and SeeD with purpose to kill the evil sorceress in the end just like Squall said in the past..... Perhaps she is not aware that someday she will be the target because Ultimecia possesses her soul.