You kind of can though. You serve cached assets and then use JavaScript to modify it for the individual user. The specific user actions can't be cached, but the rest of it can.
The "can't" comes from the fact that VLC is not going to rewrite their forum software or software forge.
Software written in PHP is in most cases frankly still abysmally slow and inefficient. Wordpress runs like 70% of the web and you can really feel it from the 1500ms+ TFFB most sites have. PhpBB is not much better. Pathetic throughput at best and it has not gotten better in decades now.
I don't know how GitLab became so disgustingly slow. But yeah, I'm not surprised bots can easily bring it to its knees.
Totally. Remember slashdot in the 1990s used to house a dynamic page on a handful of servers with horsepower dwarfed by a Nintendo Switch that had a user base capable of bringing major properties down.