Most presidents are reelected to two terms, and so if judicial terms were offset from Presidential terms by a year, they'd have:

At 0 years, 0 justices they appointed.

At 1 year, 3 justices.

At 3 years, 6 justices.

At 5-8 years (and up until 1 year into the next term), all 9 justices.

So most President get to completely stack the court, the effects just don't last as long—the judiciary becomes another weathervane political branch of government, because the two of those was too few.

(And I don't even want to consider the consequences of all those short time justices looking to their post-Court career while sitting on the bench.)