Your employee's life does not provide value to the company, their work does, and they can end that work at any time that they (or you, in most states) choose. You do not own your employees, or their future labor.

Being allowed to take out insurance on your employee's life makes about as much sense as being able to take out insurance on a hotel that you're renting a room in.

edit: clarity

How is a company to insure against a disaster wiping out an entire branch's worth of employees? That's certainly a low probability event, but many companies with single-digit locations would be in big trouble if they lost such a huge proportion of their workforce at once, which is pretty unlikely to happen naturally.

Assets are things of value that are owned by the company. It is illegal in most countries to own human beings.