Whoever used ignite has to be in range to cast it, so if you are playing a squishy and you're in a good position in a teamfight, then the enemy who ignited you would have to be diving, which exposes them to focus fire. Of course it could be a tank but then they'd need to have more enemies diving with them for that ignite to matter at all.
And yes I know assassins and divers can dive you but those champs are not too hard to kill with focus fire.
If you're playing a diver or an assassin you just have to pick better timing. You should be aware of which ults and summoner spells are on cooldown on the enemy team if you're playing a champion who is meant to clean up. If you're playing a champ who is meant to initiate the fight then you just have to accept that you're probably going to get focused. You should be able to get your job done if you pick the correct timing for your engage though.
If you get caught out of position at bad timing and focused that's just part of the game. It's always been part of the game. It means you made a mistake.