“Why do people struggle with lack of faith?”
On its own, the word “faith” is content-free. There is no such thing as generic faith. To talk about believing without asking what you are supposed to believe is like trying to order dinner at a restaurant when you’ve never been given a menu and the waiter refuses to tell you what the options are. A question so unspecific is quite impossible to answer meaningfully.
Faith always has to be in something or someone. It cannot exist in a vacuum.