It is DRM, and Amazon Prime is particularly bad, it servers 540p or similar - it is basically unwatchable (other streaming services default to 720p, which is OK, with netflix having an unofficial plugin that makes it 1080p).

And I was thinking I'll watch The Boys in nice quality on official streaming service, turned out I had to cancel Prime and download 2160p torrent. I'm still puzzling what DRM solves, in most cases it pushes me into torrenting.

Agreed. DRM makes me want to spite the company even more. Sadly, we're the minority, so it won't have any affect. Normies continue to have 7 streaming service subscriptions, one for each show, and watch them at an "automatic resolution" of 480p.

If 540p is unwatchable for you, you need glasses.

Isn't it the other way round?

Netflix when streaming to most PCs too, unless you have a specific batch of Intel CPUs, a monitor blessed with holy water, and you have counter-clockwise flow in your sinks.

It's better nowadays. All you need for 4K is Kaby Lake/Ryzen or newer (or a NVIDIA/AMD GPU from the same time or newer) and an HDCP 2.2 4K display.[1] 1080p is just practically any capable CPU/GPU with old HDCP.

1: https://help.netflix.com/en/node/23931

One alternative requires a torrent client.

This is generally because of DRM. They only support higher definitions on stricter levels of Widevine DRM, which need both software and hardware support.

That's a DRM restriction.