The same reason batteries are sold separately with toys.
Basement Watchdog builds thousands of these at a time - but they do not sell all at once.
Some units will naturally sit on the shelf longer than others.
To ensure the unit works right out of the box, we frequently purchase a limited supply of batteries and constantly rotate our stock.
This is a great replacement model. If you want a new system with a fresh battery, get the SPD-BWSPKIT instead.