Apple Watch Battery not replaceable

I received my apple watch August 2017 and towards the end of last year my watch has been giving me issues. I went the past weekend to Apple store in Canal Walk and they advised that it it my battery that is faulty. They said that they don't replace batteries and that they would just quote me on a new device. How is such an expensive device not serviceable? Something as simple as a battery that cannot be replaced! If a battery is not replaceable, how can its lifespan only be for a year and a half?