Why when I come across an Assassin I'm dead but against a tank I still have a chance of dieing?

I was playing league for a little bit and was wondering the reason. When I play an adc champion an assassin runs up to me and i have no chance to defend myself with any cc cause their and blocks damage with for example they have insane gap closing, invulnerability, and single target damage. So I understand assassins counters adc's. Then that got me thinking "i thought adc's countered tanks" because when I come across a tank, it's not like they are instantly scared and run away when they come across me. They can actually fight me and win. With items such as and they can kill me and I fear them too. So adc doesn't counter tanks? Why do tanks have all these items that counter adcs and not particularly assassins? I thought it worked in a circle of countering. Playing the adc role feels weak and dumb. Tank items can help them against adc's but why don't adc items help them against assassins?