It isn’t. Seriously. It’s just something you can practice as you do everything else, although it’d probably be harder on some harps than others. (I love the Salvi Ana or the L&H Prelude, but they arc upwards quite a bit and it might be harder to hit the right lever on them than on a Ravenna or some other less vaulted style of harp.) It’s also something you can arrange around as well, putting certain notes in one or the other hand, and even taking flips with the right hand if they are near the part where the neck dips down.
I think people just get used to not flipping them and thinking of in-piece flips as being “advanced.” And probably the best way to cope with that is to just get used to flipping from the start, to just present every piece with at least one flip and to teach people to look ahead for them, just as you might look ahead for a string.
I imagine it also helps to be an arranger. Since I’ve already arranged/composed a bunch of stuff for piano (where you learn theory without realizing it), I’m familiar with how to make things convenient for myself and what the cheapest way to get across a chromatic modulation might be with the smallest number of flips. For example, the diminished chord in “Vaga Luna” only needed one flip to get across since I avoided the F#, and I was able to avoid an in-piece flip for the Eb in the bass by setting it from the start and working around it for the rest of the piece; I only decided I wanted the E♮ back in the coda. Now on the piano, I’d definitely have put the whole chord in. Since you always have all the notes you need on that, you can harmonize much more lushly.
Anyhow, I think that in-piece lever-flipping is one of those instances of expectations becoming obstacles, where people might find them much simpler and less intimidating if they just got used to doing one or two in every piece. Seriously, they aren’t that bad. 🙂 Simply playing without buzzing is a much greater challenge.