They CAN. They don't have a GUARANTEE they will be able to do that. Even if they aren't ahead, or... they rush an mr item, they can do what you say.
This is where TEAMWORK comes in. Sure your frontline isn't as tanky, but even if it's a support braum or losing jungle shyv who only has bloodrazor and ruby crystal, they have to go in first and FORCE the enemy to use cc on THEM. It's like yesterday I was fed on renekton, but the rest of my team was behind. I was the only tank besides braum. So braum HAD to initiate so I didn't tank cc from 4 people and all their burst and dps - or I'd die in 2.5 sec and barely get off damage. Even though he couldn't tank much, if he ate a bit of cc and some basic abilities, it bought me time to get on an enemy carry while full hp and do some work. Then they couldn't burst me as fast - I could get off a full damage combo BEFORE tanking any serious damage - usually allowing us to win the fight. When no one else would initiate? We lost hard.
Point is - you shouldn't BE the frontline as an ap caster. If you and your team play it right - especially knowing YOU ARE THE DAMAGE SOURCE for your team, you won't eat the enemy's initial cc. You can position properly to open the fight, and while it's hard to ignore a sej or something in your face - they don't have the damage to solo you. They've used their initial cc - look for the chance to slip past them and focus down the carries behind them if you can blow one up. If you can't, you look to kite and trade out who is in frontline HOPEFULLY before they die. So shyv tanks a bit, then ults away since it can't be interrupted, then sup braum tanks... you get the idea. All while kiting backwards and using BASIC ABILITIES (don't waste ult on just a tank or even just 2 tanks) to whittle down enemy tank(s). All the while, you are looking to get past them when their cc is down so you can blow up the enemy carries. Also, you preferably fight by your towers - forcing the 0 armor enemy tank to soak an additional source of damage if they want to cc you.