AirTags get about a year of battery life from a 2033 in moderate/typical use cases, according to Apple. That’s 0.675 Wh (Panasonic 2032) / 1 year = 77 microwatts. Or about 7 joules/ day. That’s a lot to guarantee for production. Watch movements are also at least an order of magnitude more expensive than batteries and have a service life.