Because knowing what people need requires an expensive and error-prone system. Most people know what they need and will go buy it given the money to do so - the joy of fungible assets!
Unemployment provisioning in the United States is relatively inexpensive. I wonder if the administrative savings are being overstated, having been estimated before governments digitized such things.
UI isn't about identifying very specific needs, it's about identifying a particular easy to assess monetary need and meeting some of it: people whose lifestyle is adapted to the income provided by their job, without it, need an income roughly comparable to the take-home income from their job.
That's pretty much the easiest need assessment in public safety net programs.