The problem with armor itemization through the lens of Kayle builds

I'm a Kayle (The most true to form hybrid in the game) main, and recently i've found a build that allows me to completely shrug off magic damage, yet I have no effective answer to physical damage. When against AP comps, I build manamune>guinsoo>wits end>Maw>Deaths dance, which gives me truck loads of damage and around 160 MR plus the Maw shield and deaths dance passive. When an AD assassin like rengar, zed, or talon is the problem at hand, I usually build AP so I can make use of zhonyas, so Gunblade>Nashors>Zhonya>Guinsoo>Deathcap. The only possible other option is GA. With the AP build, only the likes of veigar or maybe a syndra or full build brand could oneshot me assuming I dont ult, while any other champion would only be able to take off 50-70% of my health in a combo. With the anti AD build, I can only budget for either GA or Zhonya, and I will still take nearly true damage from most assassins, and even after I blow my ult to cancel the bulk of their damage such as zed ult or initial rengar Q, they usually have enough damage through basic abilities to oneshot me afterward.

Why is it, then, that AD assassins should be able to oneshot anyone with no regard for how theyre itemizing while AP assassins can be almost completely negated with two items? wits end and MAW are just two of many items that I could choose from and, at the same time, the GA or Zhonya that I build against AD is usually just as effective, if not more in many cases, against AP. Pretty dumb if you ask me #makearmoritemsgreatagain #ornerflethality