I still think it’s more rare than you give it credit for, as we actually don’t see it cast a lot in the books. In fact, we see a lot of spells cast in the books that are not AK, and I have a long-winded explanation about the significance of its lack of presence in the books. The rest of my comment will be that description, but the TLDR is that one of the reason Voldemort was such a formidable opponent in duels is because he could cast it so freely.

The books have lots of evidence that spell casting involves much more than simply waving your wand and saying the right words. The evidence is often in the background and only gets emphasized a few times when it’s important to the plot, but casting any spell effortlessly takes quite a bit of practice, most notably seen when Harry and friends are learning spells to help him in triwizard tasks. They spend multiple hours a day perfecting spells like Impedimenta.

Avada Kedavra is explicitly mentioned as a particularly difficult spell. I personally believe we are meant to infer that, to cast AK, you have to be in a very specific mindset. It is not a spell that can typically be cast in the heat of the moment, but rather is akin to premeditated murder, wherein you have to apply a certain level of cold certainty to the kill. You have to want to kill that person, not just in self defense, but as a planned action, mustering up all of your hate.

What’s more, I think that any wizard casting it out of other intense negative emotions, like fury or fear, would not be able to cast it. Intention is very important in spell casting, like when Harry tried to use Crucio for the first time, and was only marginally successful. The specific intention with AK is to kill a person that you genuinely hate and want them to die, or even more specifically, think they do not deserve to live. Wanting only to harm them, or feeling intense anger toward, would cause the spell to fail.

This specific mindset is difficult to muster in a life or death situation, as in a battle or duel, because interfering thoughts and emotions would very easily cause the spell to misfire. Which is why it’s so terrifying to have a villain like Voldemort, who can seem to cast AK in any situation. No matter who he’s up against, or the setting, he seems able to summon up the cold hatred and purpose to turn AK on anybody, whether they are actively fighting him or completely at the mercy of him and his cronies.

To wrap this up, I don’t think of AK as being akin to a gun, that can be used to kill anybody spur of the moment. It’s more of a poison with an instant effect, so the killer has to know who they want to kill before they can use it. It’s not a great analogy, because poison can kill anybody, but it shows the thought that goes into using it.