Games need ultra-responsiveness because rendering frames slower is essentially blocking further user interaction, since players are expected to provide a constant stream of interaction. Being 'engaged' is essentially requiring constant feedback loops between input/output.

On the web the task of reading a webpage doesn't require constant engagement like in games. UI (should) behave in more predictable ways where animation is only there to draw association, not provide novel info. Similarly UI animations are typically (or should not be) blocking main thread responsiveness and (should be) interruptible, so even low frame rates are not breaking the experience in the same way.

But still, your point stands, its crazy what we've come to accept.