In textbooks and school, why is it that vectors are often shown to be n x 1 or 1 x n matrices? I have rarely seen other dimensioned matrices representing matrices. 

\begin{bmatrix}
 1 & 2 & 3 & 4 & 5\\
 3 & 4 & 5 & 6 & 7\\ 
 \end{bmatrix} is usually a matrix but

\begin{bmatrix}
 1 \\
 3 \\
 4 \\
 \end{bmatrix}
is usually a vector. Why?