How can I find maximum power transfer using Thevenin equivalent
It's a simple error in your reasoning.  When the load resistance is equal to the Thevenin resistance, the voltage across the load is precisely 1/2 the Thevenin voltage.

Thus,

\$P_{L,max} = \dfrac{V^2_{th}}{4R_{th}} \$

Your calculation would be correct if the entire Thevenin voltage appeared across the load but that isn't the case.