They are the only way to get non-20 cash in many areas; the ATMs that can dispense other bills are quite rare. And if you want $100 in ones you're going inside.
They are the first line of human-to-human contact with customers. They are able to sell new services or upsell existing services to customers, especially with the customer's data right in front of them. A new pleasant conversation plus "Oh by the way, did you know that you could get service ABC that would help you?" is something that an LLM or ATM can't do reliably.
There's a tremendous amount of opportunity available with well-trained tellers.