Under the current system, people aren't sure what the government is aware of. Because we get in trouble for leaving things off, we are more likely to report everything.
Make people file a tax return, but auto-populate it with the data the government already knows. And at the end include a tick box clause that essentially says "I have included everything and reviewed the autofill info, and if I have omitted anything later discovered I will be prosecuted".
20 seconds to file your taxes if there's nothing unknown. Very easy to add extra info. Covers the tax departments back of you intentionally mislead them.
The IRS information is at times comically incorrect in fact.
99.999% of the US population is not trading $20 million a year in stocks, and everyone else here is merely talking about the 95% of the population that has straightforward tax returns.