You're missing that those 80% are not the same people every year. The IRS definitely doesn't keep a database of who is married or not, nor are they scouring every counties database for changes. I'm not sure where you're getting the "they will just send you a letter" part.
Sounds fine to me. Sometimes you have a simple W-2. Sometimes you need to report life changes. "Efficiency" means those 20% figure it out when they have to.
