I can understand why UPS doesn't want to make a second delivery attempt because most of the time the second attempt probably doesn't result in a delivery either. However, from a customer perspective they may not realize they had a signature required package coming and the customer might make arrangements to be home the second time.
However, in today's day and age why can't UPS reach out and make sure someone will be present before even making the first delivery attempt? There is all kinds of technology to reach out to customers.