Booru tags are MUCH better suited to AI than natural language.
When trying to do natural language you are converting from real text to associations between words and phrases, this is very abstract and hard.
Tags reduce it back to being single words/tokens associating with each other.
When you see the tag "red hair" the model can work with red hair, but working from the tag "The women has green eyes, red hair and is waving to her friend with orange hair" is just convoluted. Simple single token tags are working with the technology not forcing it to work harder.