Well, I think the reason Kia used that 45AH battery is because it was readily available and so they didn't have to have a specially made one (which would increase cost slightly). Just my guess. But you're right: A typical lead-acid battery for an ICE vehicle gets used up so quickly because of the high demands put on it to turn over the engine, which you can imagine would not be easy especially in high compression engine situations. Plus, I imagine that the high temperature under the hood of an ICE vehicle doesn't do it much good.
Neither of those things are really a factor for an EV because the regular demands on it are low and the temperature under the hood is a fraction of what it is on an ICE vehicle. I'm always amazed at not only how relatively cool it is under my hood but also how CLEAN it is (since the bottom of the engine compartment is closed instead of open to the road, etc).
As for how long the battery that came with it lasts, mine died very shortly after I got the car, less than a week! Luckily I had already ordered the lithium replacement and had it on standby. In general I would think even a lead acid battery would last longer with an EV than with a an ICE vehicle for the reasons mentioned above. Not sure if the stock battery is a deep cycle battery, like the lithium replacement or not though. It's good that the price of these things are starting to come down. Supposedly they have a lifespan of about 10x what a lead acid battery has. That would mean 30 to 50 years, which I doubt, but I still imagine it will be in good shape at 5 or 6 years. I test it (them) every month or so with a multimeter just to make sure it's holding a good charge, etc.
Kissimmee, Florida USA
"Even 93 miles of range is 3 times farther than the average person drives in a day."