They're basically bank receptionists for old people who will type details into the same system that the general public has access to. They also handle cash for small businesses (I worked in a cafe during university and we'd regularly have to do runs into town to deposit rolls of bills and get more change to float the till)
You also need to see tellers these days to do large withdrawals (think cashier's checks) and, ironically, resolving account lockouts due to fraud. The big banks also have relationship banking if you have enough money held by them, which I understand can be very useful in certain situations.
If that's all you think tellers are then you're missing out on a lot of opportunities.
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.