The Associated Press had a fact check on Donald Trump's promise of a simplified tax form. The piece noted that the IRS has had a simplified "1040EZ" form for decades and it is not clear that the form will be any shorter or simpler with the new tax law. It did correctly point out that many fewer people will itemize their deductions, which will make filing simpler for them.
It would have been worth pointing out that the Trump administration could have made the filing process much simpler but chose not to. It could have had the IRS fill out people's tax forms for them. For the vast majority of people who take the standard deduction, the IRS already has the information necessary to determine their tax liability.
This means the IRS could fill out their forms and then send them to taxpayers for their review. If the person feels the IRS made a mistake, they correct the form with the necessary documentation. Otherwise, they accept the refund calculated by the IRS or pay the additional tax being assessed. This has been the practice in several European countries for decades.
The likely reason that Trump and the Republicans in Congress chose not to go this route is that it would wipe out H&R Block and other tax services and software companies who get tens of billions of dollars in revenue each year from people for doing their taxes. This seems the only plausible explanation since Trump and his team couldn't be that much more incompetent than the folks running tax agencies in other countries.