why is hydrogen not placed in group 1 in the periodic table?

Like an atom of Group I elements, the atom of hydrogen has 1 electron in the outermost shell. However, the properties of hydrogen deviates much from those of Group I elements. For example, Group I elements are metals but hydrogen is a non-metal. Besides, hydrogen can form H⁻, but no Group I element can form anion. Therefore, hydrogen is not a Group I element.

The properties of Group I elements deviates much from the Group I elements. This is because hydrogen can gain one electron to attain the electronic configuration of a noble gas by forming one covalent bond or by forming an H⁻ ion, but Group I elements cannot.

In most versions of the periodic table it is. Here is one example of many:
http://periodic.lanl.gov/index.shtml