Because it isn't polarized, and that can lead to a situation where something is switched "off" but there's 230V of electricity still present. Take a look at this thing:
At the bottom you have a standard European Schuko socket and Plugged into the socket is the power strip on the right. Plugged into the power strip is the lightbulb on the left that I've mounted and wired so I can measure the voltages between live, neutral and earth.
Above the power is on to the strip, but the switch in the power strip for the light off. Unsurprisingly, the lightbulb is lit off. Even less of a surprise is that turning the switch to on lights the light.
For the rest of the blog I'll remove the lightbulb and just concentrate on what voltages are present at the lightbulb holder. When off there's no voltage (or rather a tiny phantom voltage) between live and neutral (brown and blue).
But when the switch is on, the multimeter is showing 230V AC between live and neutral. So, no surprise that the light comes on when the switch is on!
The same voltage is present between live and earth. In a standard European or UK house the earth and neutral wires are joined together (called "bonded") somewhere close to where the power comes into the house. The live and neutral wires are two halves of the circuit and electricity needs to flow from one to the other for the light to come on. In this standard set up the neutral and earth have (theoretically) the same "potential" of 0V. When the light is on the idea is that electricity flows from the live to the neutral.
If you touch live and neutral you'll electrocute yourself, but also if you just touch live because the electricity will run through you to earth.
Between neutral and earth there's close to 0V of difference. The multimeter is showing tiny phantom voltages induced in the wires.
The important thing to remember is that the voltage in a home is "referenced to earth" and thus will run to earth (potentially through you) given the chance. That's why you get an electric shock if you touch a live wire. The current flows through you to earth. You do not want that to happen. In theory you could touch the neutral wire and not get electrocuted because it's at the same voltage/potential as earth: 0V. BUT DO NOT DO THIS EVER. EVER. YOU DO NOT KNOW THAT NEUTRAL IS ACTUALLY 0V.
Take a look at this; all I've done is unplugged the power strip at the bottom and plugged it back in the other way around. Suddenly, there's apparently 230V between neutral and earth, and that's with the power strip turned off!
That's because the little power switch on the strip does not cut both the live and neutral wires when turned off; it cuts off one of them. If you plug the strip in one way you might get lucky and cut live, but you can easily plug it in the other way round and find that what looks like it's neutral as actually live and has 230V. THIS IS ANOTHER REASON NEVER TO TOUCH SOMETHING YOU THINK IS NEUTRAL. Also, imagine you've turned this off and are changing the lightbulb. The same 230V is present inside the light fitting just waiting for a human finger to become a conductor!
You can see that despite there being 230V present the lightbulb does not glow. But there are dangerous voltages present. In fact, with the lightbulb present it's even worse, because the "live" and "neutral" wires coming out of the light fitting are 230V because the 230V has passed through the light and is now present on the wire on the other side of it.
(Ignore the multimeter here as it's not connected).
So, in summary:
1. DO NOT TOUCH WIRES IN HOUSEHOLD WIRING EVER.
2. Do not trust power strip switches to actually remove all dangerous voltages from a device.
3. Do unplug things when working on them (e.g. changing a lightbulb)
4. Do buy power strips which have switches that cut both wires when off (such as this Brennenstuhl power strip; not an affiliate link or anything, I just use this one in my lab.).
OK, that last one's in jest because there are reasonable criticisms of the British plug design, but it, perhaps unreasonably, bothers me that it's possible to turn something off and discover there's 230V present.