Because, as performance gaps between gens shrink, the performance boost of OC increases in relation to those. Also because sometimes you need the performance to reach certain performance "thresholds". Had a lot of fun tweaking and playing Witcher 3 at 5K recently, something not at all doable (for a solid 30 fps lock) without a good OC on my card.
So, let's say an OC nets you 10% extra performance (which is generally true, sometimes more). But the difference between your 1080 ti and the newest 2080 ti is 40%, that means that the 10% you get from an OC now puts you a quarter of the way of the difference between gens. For the other three quarters you'd have to pay £700+ (assuming you'd sell the 1080ti).
So, at the end of the day... why not? Especially on Nvidia cards, which are much less time consuming & simpler to OC than let's say Vegas.
In my case I would say my 1080ti is about 1-2% gain from any O/C I can gain from it. In terms of legacy o/c (moving slider to right). I get more from undervolting as that reduces temp based throttling, as the card will throttle at 35C, then 45C, then 55C, then 65C.
1070 palit going from 2038mhz to 2060mhz what is that in % terms?
My 1080ti goes to about 1940mhz out of the box whilst temp is rising, but will then throttle down to about 1700mhz at stock voltage curve and fan curve, aggressive fan curve gets it to go down to about 1850mhz or so. Moving power limit up and clock speed up it will peak at about 1980mhz (2ghz crashes), but then proceed to hit 70s in temperature and throttle down to mid 1900's.
Voltage curve tuning (undervolting), my most aggressive profile is 1987mhz, going up to below 55C will only drop it to 1974mhz, going above that drops it further to 1965mhz. So faster but not a huge amount faster than non voltage curve tuning, however the card is using 40-50w less power in this config and about 15C cooler in demanding games.
I discovered I can gain no clock speed between 1.031v and 1.075v, its all just extra power and heat for nothing.
I accept the lucky people can get good manual o/c, perhaps they buy a low factory o/c binned card, "AND" get lucky on the silicon lottery, but just remember these are not guaranteed o/c's.
The vendors have got really good at getting the most out of the chips out of the box.
Memory clock speed is another matter tho, no auto turbo for that stuff, so I still o/c memory.