In order to stream videos in HD from the likes of Netflix, Hulu, Amazon Video, etc., phones need to support something called Widevine Level 1 DRM. As some users on OnePlus's forums have noted, the 5T only supports Widevine Level 3 which is capable of just SD video streams.
You must be kidding me! Sounds like YouTube does HD video, but basically no other major apps do anything but SD.
For a good laugh look at the comments:
What's mind-boggling to me is why people want to watch videos like movies and TV on their phone instead of on a monitor or real TV.
How was this not noticed / pointed out in the reviews of the phone? I mean, the original Oneplus 5 has been out for many months now and nobody has pointed this out until now??