Why are the non-Canonical gospels not considered valid?

What about the other Gospels that aren’t in our canon of scripture?
Why are they not considered valid?

Ultimately, it was the bishops – the leaders of the Church – who made
the final decision but this decision was not reached in some
smoke-filled back room. It was a decision that was based upon the
experience of early Christians – people like you and me – who, in the
first century, had come to embrace certain books as worthy of being
considered inspired by God while designating other works as either
falling short of that or just downright missing the mark. This being,
said, there was no definitive list or canon of the New Testament until
the 4th century. Much of what was not included in the Canon remains
available to us in the form of what we now refer to as apocryphal
writings. Such books give us a glimpse into the mind and heart of early
Christianity and are worthwhile for study however they are not
considered inspired because of errors they contain in their presentation
of the Gospel message. Some of our Catholic Tradition can be traced to
the apocryphal Gospels. For example, the names of the parents of Mary –
Joachim and Anna – come to us from apocryphal writings such as the
Gospel of the Birth of Mary and the Proto-Gospel of James. Likewise,
have you ever wondered why images and statues of Saint Joseph often
portray him holding lilies? This is the result of a legend, included in
the apocryphal writing known as the Protoevangelion, about how Joseph
came to choose Mary as his spouse. In addition to these lovely images
and passages, we also find strange and misleading passages such as the
story from the apocryphal Gospel of Thomas in which the child Jesus gets
upset with one of his playmates and strikes him dead! This is not the
Jesus we know. In recent years, author Dan Brown has made a fortune
exploiting the lack of knowledge that most people have about the
apocryphal gospels by proposing that the Church has been involved in a
massive cover-up of these writings for centuries. Not true. I studied
them in college…they were right there on the shelves of the library.
And they are available on the Internet. See for yourself why Christians
have found them intriguing, informative in some ways, and largely
unreliable in other areas.