To get the obvious out of the way: so you can still find them when they aren't plugged in to the wall!
...but I guess you meant "why can't they use a rechargeable battery for that" - which is a reasonable question but I suspect that a regular coin-cell lithium battery is cheaper and more efficient than a similarly-sized rechargeable + the requisite charging gubbins. I've got some timer switches that I only use during the winter that are rechargeable and they lose their charge after a few months of being unplugged. Plus, the target market probably already have airtags and have a stock of the cells.
Not very green though - but the green thing to do is not buy so much of this junk.