Engineers at Google get bonuses for shaving 0.1% off of something, because at their scale that could be millions of dollars saved.
Hence protocols like HTTP/3, which exist almost entirely to optimise some Google backend by single-digit percentage points.
YouTube has had every last percent of "inefficiency" squeezed out of it, to the point where lots of users have a degraded experience.
Google famously doesn't care about user experience at all. They care about costs and their own internal KPIs, which are all tied to advertising revenue, not "video playback smoothness".
This is why Firefox was 5x slower on YouTube for years. This is why Google famously has next to zero "customer support", even if you pay them. They don't view you as a customer. You're the product.
Firefox users click on ads less. Firefox users tend to have adblock. They're not good products.
Similarly, Google is fighting a turf war with the likes of NetFlix and Apple for advertising eyeballs, so they do not want to ensure that Apple TV can play back YouTube in the best possible quality. They optimise for Chrome and Chromecast first, everything else a distant second. Got to build that walled garden!
I pay NetFlix the same amount monthly as I pay for YouTube Premium. NetFlix provides support, YouTube doesn't. NetFlix works flawlessly on every device I have, YouTube doesn't.
YouTube doesn't play 4K on my Apple TV 4K! It doesn't play 4K on my flagship Samsung TV! It downgrades my iPhone for 480p even on WiFi!
This kind of anti-consumer (anti-product?) bullshit is why Google needs to be broken up.
The vendor that makes the device, the browser, the search engine, the network protocol, and the advertising platform shouldn't also be television for half the world.
They shouldn't get to degrade the experience to benefit their browser team. They shouldn't get to slow down the experience for a competing browser. They shouldn't get to simply ignore customer complaints. Television broadcasters in most countries have to answer to an ombudsman. YouTube doesn't.
That's too much control that invites anti-competitive, anti-human behaviour. The incentives are all wrong.