Two points:
1) (and Helmut and Gaile allude to this) Some of us still don't have mature, fully developed trees. When you're in that boat, and you're researching an ancestral line, you will eventually get to a point where you consciously decide not to create that next profile, or add that next name to the data base, even though you may know it. That happens when you find yourself drifting so far away from your mainline tree that you would rather spend the available time on your more direct ancestors.
2) The U.S. is relatively young, and many people are able to work their ancestral lines back to the old country. For European immigrants from, say, late 1600s to mid-1700s, it is not unusual to find cases where the spelling and/or pronunciation of a surname changed in the new world, sometimes in multiple ways for different branches of the same family. So there are situations, when you're tracking back from an anglicized surname, in which you may not be able to verify the correct LNAB of a parent.