Oh man, I say the same thing allll the time... it's kind of unreal how difficult a time people have with this. And it's irritating as hell.
It's even worse when the enemy team has like 4 AP, or 3 AP and a crapload of cc like...

and a teammate or teammates refuse to build any MR, carry champ/lane or not... adc won't get anything at all, and mid lane waits til 4th item to get an abyssal scepter. Meanwhile they're both 0-6 1-7.
The excuse is always like "but I need more damage, otherwise I'll be useless" or "I just don't have the gold, because I'm so behind! have to build damage first to get kills/gold"
Except that it doesn't work that way, and in fact works the complete damn opposite... lol. They're already useless, and they get SUPER far behind, because they have no merc scim to break veigars stun, or amumus root/ult, or survive zyra/cho, or whatever, and keep getting murdered and one shot.
If it was me, on the adc I'd probably rush a merc scim second (vs that particular comp) and maybe even get a banshees or maw 3rd or 4th TOO. lol
Then the next excuse is "they do so much damage, the little MR won't even help..." -_-
The point is to build appropriately so you don't GET super far behind to begin with, and yes, the MR DOES in fact help lol. If you survive a single one-shot, because your maw passive proc'd (PLUS the MR itself), or your merc scim active saved you - that's one less death (feed) to the enemy, not putting them further ahead of you. On top of which, you can now react, and counter attack accordingly, since they just used all their cooldowns........ lol
And hey, guess what... that gets you gold! So you can buy more "damage" or pen cus they have 2 tanks and some zhonyas. This is how snowballing works. And it works both ways - tons of AP, tons of tank, whatever the case. They get ahead/scale easily if you don't build to counter them, you get ahead/scale easily if you do.