Because of human stupidity. Some people decided they could use liquids and a bit of chemistry to mix a few and make explosions, or use some that explode as air pressure changes. You can take liquids on flights just in small/controlled amounts because if those explode it's not going to do any serious damage.
Basically the idea is that it's better to limit liquids to a small amount like that instead of check every single one people decide to try and bring on.