resizebox doesn't resize a tabular vertically

You want \resizebox* not \resizebox the latter scales the height whereas the former scales height + depth. A tabular is vertically centred so the height is half its vertical extent.

I do not know why, but:

1) If you want to have proper dimensions, use:

\documentclass{article}
\usepackage{graphicx}

\begin{document}

\resizebox{2cm}{2cm}{\rule{6cm}{7cm}}
%\resizebox{2cm}{2cm}{\begin{tabular}{l l}A&B\\C&D\end{tabular}}
\resizebox{2cm}{2cm}{\begin{tabular}[b]{|l l|}\hline A&B\\C&D\\  \hline\end{tabular}}

\end{document}

2) If you want to see a horror, please change [b] onto [t]. I know that it is not a proper answer, but one should to see THAT! :-)