As Geezer says weeds have evolved with a range of pretty nasty chemicals in their foliage to deter plant eating creatures, slugs included. Also don't forget that a lot of the crops and flowers, that we now prize for their nutritional or aesthetic quality, have been have been selectively cross bred or if you like genetically modified by us over the centuries. This programme will have reduced the potency of the inherent deterrent of the original "weeds" that we now rely on for food and our garden blooms. Just a tip, did you know that slugs and snails love flour? Plain flour or cornflower laced with slug pellets will clear the blighters away from you pots and flower beds but does make a damn mess if it rains on it before you have chance to clean it up.