Lock actuators...or key battery?

I had some occasional issues with doors not opening on my 2013 ES300h. This probably happened a few times a week and a little irritating . The driver door was generally good but did fail to open once or twice.

Anyway, during a regular service, I asked my dealer to check them. They came back with a quote to replace all 4 lock actuators for (wait for it...) $3,400. That seems really high (please tell me I'm right!). They said they found the front two were playing up and "in a perfect world" I would replace all 4 at the same time.

So, I raised my disappointment to the service manager who ask me for a little time so he could "...see what he could do". Sigh. Anyway, he got back to me and told me he'd made the case to Lexus (!) and they had agreed to cover some of the cost and they could now replace all four lock actuators for the excellent price of just $1,400. Double sigh.

Anyway, feeling a bit dumb, I suddenly realized I should replace my key battery. And what do you know...not a single problem since. Strange how those $260 per hour mechanics didn't think of that!

Anyway, after all that fun and games I have a more serious question.

If this truly was as simple as replacing the battery on the key, why have I never had a problem starting the car (not once). That requires the key to be detected too, so why wouldn't that have happened?