APC feels extremely weak

ADC is protected by everyone in their team. Locket, knights vow, relic shield, and etc. ADC also has many self protecting items such as Death Dance, Life Steal, Edge of Night, etc. ADC has a huge burst with static shiv + crit proc and has a very powerful consistent damage. ADC has literally no weakness in teamfights. Games are already decided by which side having the better ADC.

APC? Well they don't really have much protective items. Sure there is locket, relic shield and etc, but there is no reason to use it on a APC who deals less consistent damage which are not even reliable. APCs are usually limited to their low range to burst, and where tanks are always on their face, as result, it is impossible for APCs to kill the enemy ADC.

What is the point of APC? Armor pen % is already really powerful so enemy building armor won't be that much of a problem. Life Steal is broken. APC can't reach the enemy ADC in teamfights so their burst is useless. In the end APCs are also just peeling for their team's ADC which is really stupid and the whole game should not be decided on who is better at protecting the ADC/has the better adc.

TLDR: APC's point of having huge burst to burst carries are not put to practice due to front line tanks being too tanky and always appear in front of APC's face. As result APC's that can access the backline are poweful(Ori), but most are not able to do so. In order to access the backline, it requires to go through the front line, for this ADCs does the job best.