Sure there is. If they don't do it, someone else will be able to outcompete on price, eventually at least.
And yes, most of our history is full of examples of newer technologies lowering cost, that's kind of the whole reason that or civilization is more advanced (materially) than it was in the past.
There's also the "number of purchases * purchase price" equation which means you can make more money by reducing price if it means (enough) more people by the product, or buy more of them.
Amazon has been quite the monopolistic company lately. Just because a company started winning by outdoing the competition doesn't mean that will continue to be their only way of succeeding. Even the most principled companies will adopt a few anticompetitive practices here and there when they get big.
I think with Amazon building out their delivery and subscription options, they can hold back a lot of pressure to share the savings from automation with consumers.
Right now they have no need to lower prices. They can hold their lower production costs as a threat to any competitor who might be tempted to enter a price war, without ever needing to actually lower their prices.
And yes, most of our history is full of examples of newer technologies lowering cost, that's kind of the whole reason that or civilization is more advanced (materially) than it was in the past.