Because in the Spanish sentence, the person is not doing the liking. It's the thing that does the pleasing. The Spanish verb gustar works more like "to please" or "to be pleasing to" in English: the cool thing is the subject and the person is the (indirect) object.
- Estos vaqueros no me gustan. - These jeanse don't please me. = I don't like these jeans.
For sentence with gustar (and other verbs that function similarly), it's more common to use the word order object-verb-subject, so it might look somewhat confusing.