Probably because they have this thing called window of vulnerability.
Jokes aside, they've had nearly their entire item list nerfed in the mage 'update', then several of the prominent ones were nerfed regarding some fringe abuse cases. They can't snowball as easily, aren't as mobile, and most are limited by mana; I most as a few have too low of costs and/or too high of mana regeneration that makes this limitation negligible. A few examples that come to mind would be orianna (she actually got a buff of reduced mana costs), xerath & swain can both restore mana by farming, fiddlesticks since his playstyle is often to wait for an all-in; fizz & ekko & camile also rarely have mana issues unless you're constantly spamming to farm, though they're also more assassin styles.