It's not a bad question, these units of measurements are always a bit confusing. You can similarly ask why for humans, rubbing a balloon is harmless, although that builds up 30 kV of static electricity, while touching a 230 V power socket can kill you.

Voltage is merely the "pressure" that charged particles experience. Voltage alone tells you nothing about how much charge is actually available once electricity is allowed to flow. And that's where the harm comes from. For static electricity, when you touch something, you get maybe a microcoulomb, once, and its gone. For a power socket, you get up to 16 coulombs per second continuously.

As a kid, the alliterative mnemonic we were taught was "current kills".

"It's the volts that jolt but the mils [milliamps] that kills."

thats all theory thing is that I mess with large two volt(nominal) storage cells,the largest are over 250lbs and sit like dumb beasts,waiting to oblige anyones low voltage requests,hundreds of amps on tap be nothing to bolt ,some nice shiny copper handles to the terminals and mist them down with some warm salt water I also mess around with microscopes,and compared to bugs,humans are very poorly made,so many tiny things are flawless living perfection,and some like wolf(jumping) spiders are smart,smart enough that they see us seeing them,and are ok with that one thing that I have observed that plays into the static electricity thing,is that many of the tiny critters that I watch,are impecably clean,no dust or dirt on them at all,perfectly clean,unlike a human finger,which is one zillabutt uggly thing,under magnification

are you ok man

Hence the saying 'it's the volts that jolt and the mills that kill'.

I've usually heard "volts hurt, amps kill".

But can e.g. 3V DC kill? Perhaps by using the body's resistance, but I have the idea that the effect would be different from say 220V AC, which affects the nerves.

Not generally, remember Ohm's Law I = V/R. Internally the body has a resistance of ~300 Ohms as a rough rule while our skin is 1000-10000 depending on the condition and contact area involved.

So 3V isn't going to pose any real risk unless it's applied internally and right across a critical nerve leading to your heart or a muscle directly on the heart. For reference pacemakers are generally set to 2-3 volts. Applied externally up to ~12V is generally considered low enough voltage there's a low risk of truly adverse effects.

As shown in StyroPyro's video of him laying across dozens of car batteries (and then shorting them through various things) https://www.youtube.com/watch?v=ywaTX-nLm6Y

His entire rig at one point delivered >80k amps, but he's fine.

Speaking of, he also had this great video on lethality of electricity.

https://www.youtube.com/watch?v=BGD-oSwJv3E

You'd have to really try.

10 milliamps across your heart can kill but using Ohm's law we can calculate 3V / 0.010 A to get a resistance of 300 ohms. This means you're probably still going to have a bad time if you apply it directly across your heart during open-heart surgery but other than that 3 volts just isn't enough to drive a lethal current through your skin.

Which is why if caught in a lightning storm you should crouch with feet together and why I try very hard to only use one hand when doing something that might have the potential to shock me.

Depends how it's applied and what's sourcing it. 3v does basically nothing to dry skin, but would be quite bad on wires implanted in your chest across your heart.

When they fly, there's no current. They just have potential compared to ground. Also presumably their electrical charge is very low and there's going to be hardly any amps when they discharge.

In the same vein, if your carpet gives you a static shock, that's likely going to be thousands of volts. But obviously there isn't actually a lot of energy stored (all you did was convert some friction), so there's next to no amperes, little work the electricity can do, and thus no harm.

For many of the same reasons that birds can land on high voltage lines without risk of being electrocuted. A flying insect has stored voltage with no path to ground, or any point with low resistance and lower potential.

When you hit a flying insect with a zapper you are supplying a high potential and low potential electrode. The insects body completes the circuit and the stored voltage is routed through the insect, rendering it a flightless blob of goo.