None of the US carriers have a weight limit for carry-ons as the cabin crew contracts do provide no obligation for them to lift any carry-on if necessary.
That is why they offer a gate check service for those who can't lift their own baggage into the overhead storage compartments.

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On the other hand carriers like QR ( who limits at 7kgs ), have their crew assist their passengers, lift their baggage for them if necessary, etc...
Obviously, there are also some other carriers such as Etihad, who want to grab extra money from overweight carry-on baggage by weighing carry-ons and when it's over the limit, charge the passenger for that piece of carry-on to be it checked on hold.