Most often it's a hardware limitation because the cables/adapters use the MCDP2900 converter chip inside, even when they advertise HDMI2.1 support. That's the same chip inside the built-in HDMI port of the new MacBook Pro and its datasheet [0] says it only supports up to 60hz

That chip is also the reason for a lot of support emails I'm getting on Lunar (https://lunar.fyi/) because it seems to break DDC/CI and hardware brightness control stops working through cables and ports that use it.

[0] https://media.digikey.com/pdf/Data%20Sheets/MegaChips%20PDFs...

Same with Macs not supporting display port daisy chaining over USB C (not Thunderbolt), it's purely a software limitation.