I have no idea what matches you guys are playing. Every match actual tanks are getting blown up left and right while champions like LeBlanc and Zed take no damage and one-shot absolutely anyone.
Actual tanks early on are not tanky and get blown up. Lategame it's all about whether you have a champion who can deal DPS. Literally a half-decent BotRK user was enough every single time. Few exceptions exist, such as TK who being unkillable yet one comboing people isn't an ARAM specific thing. If you have an issue with Tanks lategame, you either got unlucky during champ select (welcome to ARAM where the R stands for Random), were dumb during champ select (stop taking a 4th poke mage just because you like 'hurr durr dmg', yes I know you guys are out there), or refused to build the right thing (I know peanut brains get more dopamine when they see one big number, but trust me, 205 is bigger than 235).
Just yesterday I spectated my girlfriend during ARAM Clash. She complained she deals no damage. Enemy had 2 tanks and 2 bruisers. Told her to build Liandry and loudly shouted the damage numbers to her that popped from the burn.
It's bruisers who I see becoming an absolute menace. Getting a tank item, then doing the usual Bruiser thing where they just lifesteal to stay on 10273 health in 1v5.
Hydra is just nuts, especially on assasisns. Eclipse + Hydra and they are basically unkillable for the duration they are dealing damage.