Because... there's too damn much defensive itemization, buffing of defensive spells (like janna shield and ult), and not enough damage on other classes besides adc right now. In seasons past, ori combo would have solo deleted kog before his team could react - no further damage needed from anyone else on the team. So you are correct that in the current meta it would be ridiculous to expect kog to die there; however, it is even more ridiculous to accept this meta as OK/tolerable in any way shape or form.
The boards is 1/2 posts about adc's and enchanter supports needing nerfs or about cho needing nerfs. As we can see, adc's in situations like that make better tanks than a fed 6 item cho'gath. People have been calling for months now (some much longer) to fix the numerous things wrong with league. Inherent scaling imbalance. Insane braindead bully potential of some champs; like when they are at their weakest, the match another champ when that second champ is at it's strongest point in the game (ahem... cough cough fiora cough). Etc. etc. Adc's and their items, enchanter supports and their items, and tanks and their items (thornmail, etc.) are by no means the ONLY issues, but they are the most visible. The fact is, a number of things have needed fixing for MANY patches, yet remain untouched either always or for a very long time before being nerfed (and probably given a compensatory buff that might even make the item or champ more op than before). Fixes could be simple adjustments often times and adjusted more from there, but instead we get long waits for some elaborate "solution" that ends up not working half the time and results in 10 more adjustments before they basically do what people who "know nothing" in low elo (like me) said should be done all along.
Example? Tank ekko. Spend half a season trying to balance him; settled on decreasing his base ad. What did many people of various elo's say to do all along? Decrease his base damage output. Wow - so hard right? Who would have thought of such a complex fix? Simple solutions like this are out there for many other issues, yet they don't get tried or are only used after 10 other things have failed and people have had to put up with a broken champ or item ruining the game for months. And it's only gotten worse over time. Having played for many seasons now, I would point back to around the time of the juggernaut rework as the origin of the balance team going downhill. Since then, there's almost always been one thing or another (an item(s), a champ(s), a class of champs, a role) that has carried too much power in deciding the outcome of the game compared to everything else. It's like Wickd put it - if I outplay someone in a matchup, but they win just because they have a certain item or champ, then there is generally something wrong with the balance of that item or champ.
