Because actual authorship was simply not considered all that important at the time.
On what evidence do you base this claim?
Psuedepigraphs are forgeries no matter how you want to spin it. The purpose of these were to decieve. If they were not to deceive they would not have been falsely attributed to start with. The attribution to some one else or the appending to earlier documents was clearly intended to deceive.
Your comment is spectacularly wrong.
Galen (ca. 129-ca 200CE) found it necessary to write a book telling how to tell his work from forgeries.
Even Christians thought it was bad form. The church father Tertullian(ca. 160-ca. 225) told a story of how the forger of 3 Corinthians was duly convicted by the ecclesiastical authority for composing this letter and falsely attributing it to Paul.
Source is The Christian Delusion: Why Faith Fails, edited by John Loftus, article The Bible and Modern Scholarship, by Paul Tobin page 168.
The claim that authorship was not important and forgeries were well tolerated is an apologist lie. Unless you can provide some sort of proof to back this up, I will have to go with Tertullian and Galen.
Facts don't lie or have an agenda. Facts are just facts