Probably because almost every other safety decision in a cars design is focused on the occupants.

those reinforced bars protecting the passenger: Do you think they care if they mean that any car hitting the side of the car suffers more damage due to hitting a more solid structure?

They want to sell the cars, thus they likely want the cars priorities to be somewhat in line with the buyer. They buyer doesn't care all much about the toddler in the other car except in a philosophical sense. They care about the toddler in their own car. The person is not the priority of the seller or the buyer.

In terms of liability it makes sense to try to make sure that the accident remains legally the fault of the other party no matter the number of deaths and the law rarely accepts intentionally harming one person who wasn't at fault in order to avoid an accident and spare the lives of a car with 2 people who were at fault themselves.

HungryHobo gave good arguments from tradition and liability; here's an argument from utility:

Google's cars are up over a million autonomously-driven km without an accident. That's not proof that they're safer than the average human-driven car (something like 2 accidents per million km in the US?) but it's mounting evidence. If car AI written to prioritize its passengers turns out to still be an order of magnitude safer for third parties than human drivers, then the direct benefit of optimizing for total safety may be outweighed by the indirect benefit of optimizing for own-passenger safety and thereby enticing more rapid adoption of the technology.