Because they have low evaporation temperatures and low specific heat capacities. So it takes very little energy to raise their temperature and you don’t have to raise it by much to get it to evaporate.
Compared to what? Water?
Most liquids have a vapor pressure at room temperature. The vapor pressure for alcohol is higher than that of water. There are other substances that have a higher vapor pressure than alcohol.
Also recall that a liquid absorbs energy when going from the liquid state to the vapor state. This is why alcohol feels cold when you dab it on your skin… it is rapidly transitioning from the liquid state to the vapor state, and it absorbs energy (heat) from its surroundings when it goes though the phase change. Your skin is part of this environment, hence your skin temperature will decrease during the phase change.
Also, the vapor pressure is higher because the molecules aren’t as attracted to each other as water molecules are. Water molecules are polar. Oxygen atoms are attracted to hydrogen atoms in other molecules. I believe alcohol is slightly polar because of the -O-H group, and that is why it’s a liquid at room temperature unlike the similar alkanes (methane, ethane, propane, etc.), but it’s less polar than water. My chemistry education is not extensive, so I may be slightly off, but that’s the gist.
Or, they just do. All liquids eventually will evaporate. Some do it quite rapidly. Others, very slowly. Ever see a spot of motor oil stay there forever? It’s about the structure of the molecules. As liquids, they’re always moving. How easily they are able to bounce off each other and break free from the group is connected to their structure, their relative attraction to each other, the amount of energy they need in order to escape into the atmosphere, etc.
Snail boy is spot on.
barely polar, and low molecular weight = Teeny molecule which takes very little energy to evaporate.
Furthermore, some heavier alcohols, including butanol and up, have a higher boiling point than water as can be seen on this page. This necessarily means that they have a lower vapor pressure than water at 100 degrees Celsius. It does not necessary mean that they have a lower vapor pressure at room temperature, and I couldn’t find a site that listed room temperature vapor pressures of anything other than methanol and ethanol, but I’d be surprised if none were lower. I believe some complex alcohols are actually solids at room temperature, but I can’t find any proof.
Also, another thing to think about is that air already contains some water vapor in it. A higher humidity means the air is closer to being saturated with water and liquid water will evaporate more slowly. Of course air doesn’t normally have alcohol in it, so liquid alcohol can evaporate more freely.