All too often, clients I have that waited until the end of March or so to file (even if they had not been waiting for information), have found that they were the victim of identity theft.
Somehow, someone was able to fake their SSN and file a return. So that IRS thought my client had already filed. When s/he/they had not.
I'm sure you've heard the saying "First come, first served." If someone submits a return to IRS and it successfully passes through some checks, then the return is considered filed for that SSN. And then you have to paper file, submit forms to IRS alleging identity theft, and for many years you will receive and Identity Theft PIN which must be used to then e-file your return.
Yes, it's a lot of work and a nuisance.
IRS does take major steps to try and curb fraudulent returns. That is one of the reasons why I and many other preparers must now ask for your driver's license.
But the best way to head off the fraudsters?
File as early as you can.