Why is methane tetrahedral ?

The number of bonds does not determine the shape.
The number of centres of negative charge does. This includes:

Xenon(iv) fluoride is a common square planar molecule. The central Xe atom shares four of its eight outer-shell electrons to form bonds with just as many fluorine atoms. The remaining four electrons are two lone pairs. In total, the central atom is surrounded by 6 centres of negative charge. In order to achieve maximum separation, the four F are arranged in a square shape on the same axis and the two lone pairs align perpendicularly.

When a carbon atoms bonds with four hydrogen atoms to form methane, all its outer-shell electrons are involved in bonding. There are no lone pairs, so the central atoms is actually surrounded by four centres of negative charge. In this case, maximum separation is achieved by a tetrahedral arrangement.