Highkey exists because lowkey was so successful. Once people were saying "I lowkey like it" to mean a bit, or secretly, they needed a word for the opposite: a lot, and openly. "I highkey love this song" says it plainly.

The term is older than the slang. In photography and film, a high-key image is bright and even, with little contrast between light and dark. Dictionary.com dates that sense to 1915–20. The slang meaning, which Dictionary.com glosses as "intense" and "out in the open", came much later, as the mirror image of lowkey, whose slang adverb only appeared in the late 2000s.

It never caught up with its sibling. In our judgement lowkey is now a basic part of casual English, while highkey is used more selectively, often for comic contrast ("lowkey tired, highkey ready for the weekend"). That's common for words formed as opposites. The original is useful; the opposite is mostly useful for the joke.

Still, there's a real need behind it. Lowkey keeps a way out; highkey drops it. Saying you're "highkey proud" of someone commits you, with no hedging, which is sometimes the point.