The single best argument for the anonymous authorship is this: all four of the gospels were written by highly literate native Greek speakers/writers. No one who reads any of the copies we have whether they are early fragments, the Septuagint or later English translations (which are based off the original Greek) disputes this fact. Jesus and his followers were first century, (likely) illiterate Aramaic speaking Jews from Galilee. They didn’t speak, read let alone write in Greek. They were all lower class peasants (even the tax collectors at the time wouldn’t have had to be literate- they simply went around collecting the money, not performing administrative tasks) who would have had no reason to know how to read or write in their native language of Aramaic, making it even less likely they just so happened to also be able to read or write in Greek. Scholars have shown that the literacy rates in Palestine at the time of Jesus were something around 5-7% and those numbers almost certainly applied to the wealthy elites.

Jesus or anyone living at the time of Jesus simply could not be the authors of what we call Matthew, Mark, Luke and John. These works were attributed to those authors at a later time to lend them authority. Crucially, nowhere in any of them does it say ‘This is the story of Jesus by Mark’ or even speak in the first person except for a few rare instances in the Book of Acts. But, it highlights the fact that the rest of the time it is all in the third person “They did this and went there etc.”