Here's why:
* Compound words are formed by combining two or more words, often creating a new meaning. Examples include "sunlight," "fireplace," and "upstairs."
* "Horseback riding" is a phrasal noun - it's a noun phrase that acts as a single unit. The individual words "horseback" and "riding" retain their separate meanings but work together to describe the act of riding a horse.
While it's not a compound word, it's definitely a common and understandable term for traveling on horseback.