AI is a wide field
AI is a wide field that goes far beyond machine learning, deep learning, neural networks, etc. In some of these fields, the programming language does not matter at all (except for speed issues), so using Lisp would certainly not be an issue there.
In search or AI planning, for instance, standardised and commonly used languages, like C++ and Java, are often the first choice, because they are fast (in particular C++) and because many software projects like planning systems are open source, using a commonly used language is likely to get more feedback or contributions. On the other hand, Common Lisp ecosystem is welcoming, the language is also standardised, and there is a LONG history in AI research using Lisp.
I am only aware of one single planner that is written in Lisp. Just to give some impression about the role of the choice of the programming language in this field of AI, I'll give a list of some of the best-known and therefore most-important planners:
This is probably the best-known classical planning system, which is written in C++ and some parts (pre-processing) in Python.
Together with Fast-Downward, this is the classical planning system everyone knows. It's written in C.
This is one of the best-known partial-order causal link (POCL) planning systems. It's written in C++.
This is the best-known HTN (hierarchical) planning system. There are two versions: SHOP and SHOP2. The original versions have been written in Lisp. Newer versions (called JSHOP and JSHOP2) have been written in Java. Pyshop is a further SHOP variant written in Python.
This is another well-known HTN (and hybrid) planning system. There are different versions of the planner, PANDA1 and PANDA2, which are written in Java. PANDA3 is written primarily in Java, with some parts being in Scala.
These were just some of the best-known planning systems that came to my mind. More recent ones can be retrieved from the International Planning Competitions (IPCs), which take place every two years. The competing planners' codes are published open-source (for a few years).