Ok kiddo,time to explain;
Last ar and leona's abilities aren't spammable,thus having high base but weak scalings.
Now let's compare it to an AP assassin with high scalings liek kassadin.
Leona Q 40/70/100/130/160 plus 30% ap on 11/10/9/8/7 cooldown.
Leona W 60/110/160/220/260 plus 40% ap on 14 cooldown.
Leona E 60/100/140/180/220 plus 40% ap on 13/12/11/10/9 cooldown.
Alistar Q 60/105/150/195/250 plus 50% ap on 17/16/15/14/13 cooldown.
Alistar W 55/110/165/220/275 plus 70% ap on 14/13/12/11/10 cooldown.
Kassadin Q 70/95/110/135/170 plus 70% ap on 9 cooldown plus a magic dmg shield(plus kind of cc)
Kassadin E 80/105/130/155/180 plus 70% ap plus soft cc AOE on 6 cooldown.
As you can see,tanks have high base dmg and some scalings so they can still deal SOME damage while building tanky,while kassadin is an assassin with high scaling,so you would by AP and pretty low cooldown at all ranks,don't forget the overpowered ultimate.
Don't forget some mana scalings.
Yes compared to mages they are pretty low but most mages need to hit SKILLSHOTS and are IMMOBILE while you aren't.