The source of immersing before prayer or studying Torah was an enactment by Ezra for the purpose of limiting conjugal relations between a husband and wife by requiring immersion if they had a seminal emission during / after relations. The original enactment also applied to women and to involuntary emissions. It had nothing to do with ritual purity, but was solely for the above purpose, as the person may still be ritually impure after the immersion - for instance a woman who became niddah after relations and had a seminal emission is still niddah after immersing.
However the enactment was canceled in a later generation due to the peoples inability to keep it, and now immersion before prayer is simply a minhag that some men have taken upon themselves.
I would assume that women don't do it because there's no requirement for them to and why would they want to take on a minhag that's quite frankly so difficult, especially when it was originally canceled specifically because it was difficult?