The reason a lot of times tanks do what feels like a lot of damage by end game (and add lane poke to tanks that are in lane to this) is that teams are focusing everyone but them, they simply have a lot more time to do damage while being ignored than others.
You can't honestly tell me you're gonna target Ornn when Cait is next to him, then you're gonna have to try to catch cait while doing so, and she's doing more running than damage if you're doing it right, which is where tanks get all their damage.
The other thing that I did NOT mention, is that at lower ranks a lot of people put literal 0 gold into defensive items, they deserve to melt. It's like bringing in a lvl 18 with scaled damage off lvls, spells, and maybe some tiny bit off items and rolling them onto a same level champion with no items. Yeah, they're gonna hurt. But that's why 0 defense builds should have front lines protecting them.
TL;DR It's a team game, tanks are low priority focus in fights, you need to both keep them off of you while targeting others, that's practically the nature of league itself. Does Ornn do a lot of damage? Relative to others, not even remotely. If you leave him to do whatever he wants, yeah it'll add up.
Personally, I'm more concerned about his CC (and CC immunity on W) than damage when I'm facing him. When I see a fed ADC then I'll worry about damage.