Why AP Caster are so behind ?

I was wondering, i'm playing AP Caster (not assassin) on mid lane for over 3-4 years now, and they have never been so weak vs AD, AD Tank, Tank ? AD have easy combination with Qss malmortus and most armors but AP Caster have only zhonyas

AD Assassin, AD Tank = Regen, lethality, most have mobility, speed move or even dashs, high builds changes without losing dmg, lifesteal, critical hits, attack speed, easy combination with armors (not loosing efficiency), anti-AP Items (qss, malmortus, the new cape), easy bruiser, cooldowns redution

Tank = Well tank are meta, they are tanky as f*ck but its their roles, now with heavy cc and easy aoe damage, easy bruiser, kiteable

Ap Caster = one anti-AD item, no armor combination worth it (loosing way too much efficiency, or at the cost of most dmg), low mobility for most, cooldowns, cooldowns redution, hitboxes ... just wrong (seen many spells not applying passives or even with ennemies walking trough lux's Q or mismatched visuals), squishy (and useless if tanky), almost 0 dmg per %hp, no more spellvamp

My main is lux, i also play malz sometimes, they got so much behind, even the shield of malz is now useless, it get canceled by any spell any passive any auto attack even the ghosts from a support item and it doesnt cancel any effects (if braum punch me, i still get the passive, if brand use his column, still get the passive, if i get slowed, still cancel the shield without taking dmg) and lux, i see people walking trough her spells all the time, visuals are still bugged, no more room for most Ap Caster because they are easy targets that need heavy protection and they can't take down any tanks or AD off-tanks in a burst so they need their CD all time, they can't 1v1 almost everytime and can't afford loosing farm/lane/tower/cs or they will be useless the rest of the game...