Because when people say "language" they usually imply certain capabilities we have not yet observed in other animals, especially when talking about what humans

Because when people say "language" they usually imply certain capabilities we have not yet observed in other animals, especially when talking about what humans have that other animals don't. It's similar to being able to use tools (which we've seen in many species) versus being able to create tools (which we've seen e.g. in certain corvids). The former is a significant first step but the later is what proves a certain amount of abstract thinking beyond the immediate here and now that goes far beyond mere instinct.