The point of paying everyone, or at least every adult, as that it's a lot easier to check for double-dipping (especially later when people know you will pursue them after finding out), than to spend weeks or months checking for all the zillions of corner cases and rules and demanding complex proof needed in ordinary benefits / social security schemes.

In the UK it is known, by government studies, that a huge amount of money that people are entitled to in benefits is never claimed because people either don't know about it or are put off by bureaucratic hurdles. Government officials generally don't draw people's attention to what they are entitled too, and often make mistakes, occasionally fatal (people have starved to death). And some people get what they are entitled to only by appealing bad decisions, with the appeal success rate being rather high.

Right now, especially for people such as those who are self-employed or with complex finances (savings and debt at the same time, etc), proving elegibility to benefits is very difficult, especially if done from scratch when they don't have well organised documentation to hand. (Normally it's a once-a-year thing, to file taxes.)

But the need for urgent finance in the current crisis is the same for everyone without enough cash.

Some self-employed or gig economy people are panicking about affording food right now, and retaining shelter; let alone bills, and that's a terrible situation to suddenly happen, especially to large numbers of people.

What I would advocate for, in a crisis such as this, is helicopter money to everyone at first, and then gradually introduce more nuanced systems that take a bit longer, and give people time to plan for. So that at least nearly everyone can be safe with regards to food and shelter in the short term.