A mono sound is only possible if both sides contain the same content in time and frequency, in the case of a two speaker set-up. When copying across the EQ for the left-panned track and shifting the frequency of all the filters (see Waves Q10 in GIF) you are changing the content of the right side - Just like one would use different amplifier settings on the left and right guitars to achieve a 'bigger' sound. As the frequencies change in the right speaker, more width is achieved (see stereo meter, bottom right of GIF).
There are countless other ways of making a mono sound stereo, but my personal favourite is using this shifting EQ or using JST's Stereo Widener tool. Although that can be a little overly synthetic-sounding, especially on very busy sources.