It costs your employer about 30% more to employ you than what you earn (approximately) - so hiring someone for 75k will usually cost a company somewhere around 100k.

I didn't say they would pay $100k. I said it would cost probably $100k including benefits - a full time employee cost isn't just their salary or hourly wage. There's lots of overhead cost to employ a person at a large company. Also keep in mind this is for a Canadian, so don't be thinking in USD. In CAD in the Toronto area (for example), it isn't unreasonable to think even a first line phone based customer service agent salary would be between $65-70k, then the employee expenses on top of that.

It’s probably half that, but a chatbot can serve thousands of users whereas an employee can manage a few at a time.

Doesn't matter to their shareholders. They'll have already made their money.