Why is 6k so powerful, with no itemized counter options?


The counter is called Ga and CC
Yes, because CC is something you can buy, and totally doesn't equally counter literally every other form of damage in the game equally well (ok besides dots, but that's like 3 champions)
A GA doesn't do much to actually counter these items alone. Yes, it gives you some armor, which is reduced to 33, and you can pretty much assume all the gold cost of the MR is pretty much wasted if you're trying to build against the person who bought botrk and LW, but let's assume that you do.
You die and revive. Your abilities are now off CD, but so are theirs, and they really didn't lose any damage between your first 100% HP, and your now 700 or 30% HP. With the exception of the 100 damage from the botrk active...
How does GA and CC counter Grievous Wounds though?

magic resist + health and you get a linear decrease in damage
No, I mean the counter to people who are building AD DPS is to burst them.
Except riot has gone out of their way to make it so that your build doesn't determine if you have burst, rather burst power is determined by the champion.
Then on top of that, you're also assuming you have viable AP ratios to even utilize deathcap to burst them down. Now you could buy other options, but really none of those do you much justice either, because it's again more about the champion you pick not the items you buy... except for offensive AD itemization, in which case, you can rely more on the power of the items over the actual champion. After all, AD is just orange AP at this point.
Offers nothing but a stat stick. Granted a powerful stat stick but a stat stick none the less