They are silly, silly people. On what evidence do they make such a claim...other than John espouses a theology they don't particularly like?
Also, other Gospels were excluded, likely in order to make the theology what the authorities wanted it to be.
Those "other Gospels" of which you speak are late. I don't believe a single one was written in the 1st century--and some were composed in the 3rd or 4th. So I find it a little odd that some of the same scholars who are always clamoring about how the earliest writings are the most reliable, are nevertheless the first to prefer the Gnostic writings over Paul and the Synoptics.
Here's a different interpretation. The four evangelists were either Apostles or companions of the Apostles who actually went around with Jesus and knew what they were talking about. The later writers, with no connection to the people who were there, made stuff up that fit their particular Gnostic theology but which twisted what actually happened.
Again, if scholars want to be consistent on this, then it's precisely the Synoptics and Paul that should give more weight than, say, the Gospel of Judas and all that other nonsense.
Which, of course, it's been well established that they were not.