On one hand Facebook wants to entice more users to sign up, and the users need to feel safe about it.
On the other hand it wants to cater to its customers (the advertisers, its owners, and soon the stock market) to stay with Facebook and grow their spending there. In terms of advertisers it’s partly about providing information about its users, for targeting.
Many users and heavy advertising means a lot of revenue and a high valuation.
If passive accounts would be deleted automatically, the amount of users would decrease, which is not in Facebook’s interest, so how far will Facebook go?