From what I can gather they are of the attitude "it was fine before so I shouldn't have to manage it locally". They resent the idea they might need to cap their speeds, losing a little in the process.
As I posted there, they've been lucky, I've always had to use QoS on Zen, Plusnet, and the multitude of others ISPs I've used over the years to manage bloat under load.
I agree an ISP should try to throttle downloads to never exceed the pipe (and they do, its in their interest not to waste backhaul capacity), but as others have posted, keeping the buffers small/empty would be detrimental to an ISP and customers speed overall. There has be a balance to allow heavy downloaders to get full benefit, while people needing low latency use local QoS to smooth out the experience. Also, upstream QoS can only be managed locally, as you are the one sending, which is why the BT HG612 config by default chops ~1Mbit off upstream.
I'm in both camps, which is why I consistently pay for the fastest broadband I can get and spread the load over 5G too so even heavy downloads rarely max out my primary connection these days.