Why, in Beamer, letters written in math mode look as written in normal mode

Question

I'm making a presentation with beamer and have noticed that letters written in math mode look exactly the same as letters written in normal mode; in other words $K$ looks in the PDF exactly like K, while when you write a paper using, for example, article letters in math mode look italicized. Why is this happening? In case you wonder, my preamble is this:

\documentclass{beamer}

\usetheme{AnnArbor}


\usepackage[latin1]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[spanish]{babel}
\usepackage{amssymb}
\usepackage{amsthm}
\usepackage{placeins}
\usepackage{amsmath}
\usepackage{amscd}
\usepackage{graphicx}

\newtheorem{defi}{Definici\'on}
\newtheorem{ejem}{Ejemplo}
\newtheorem{teo}{Teorema} 

Thanks for the help.