Wrestling isn't often seen as a martial art because it has been primarily pushed as a sport - much in the same way most people don't consider archery, javelin, or shot put, as war arts, though those are clearly origins for those sports.
However, one of the benefits of having achieved mainstream status is that it is not in danger of dying out or losing a good network of skilled practitioners.
One can, of course, find subsets of wrestling which are more biased towards their combative origins (for example, some Catch Wrestling styles) which also have the same problems a lot of martial arts have - with tactics or methods that are not designed for sport, it's harder to get regular support and practice to use them, and so it becomes about specialized training.
Of course, anyone looking for combative training can easily see the direct value, whether or not it tends to get promoted as such. (Likewise, actual traditional combative arts usually are a mix of weapons, strikes and grappling, and loss of the full art is often what removes their effectiveness).