Stereo usually has different signals being output to the L and R channels. This assumes the listener will be seated in the centre at all times for the best soundstage. If you listen to some stereo recordings the channels one side may appear to be dominant which isn’t practical if not seated dead centre.
Making the L & R output of the system in Mono allows 100% of the signal to be output to each speaker. It’s the same effect of listening to music with headphones and the soundstage is centred in your head.
For further reading the search could include “ 2 channel Mono”. Many audio manufacturers are selling Mono Block Amplifiers in pairs for this Mono sound stage.