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Because it's a tad easier to configure the UHK via Agent than QMK keyboards. http://ultimatehackingkeyboard.github.io/agent/

There are ways to do this with QMK as well.

I believe you, but I don't think they come close to Agent.

They do because they work on more than one keyboard.

Then show me a QMK GUI configurator that is as intuitive and powerful as Agent.

Massdrop has a really nice one.