MIDI mergers USED to be very common. But, Keyboardists don't need them anymore, because all of their gear is designed with much deeper MIDI implementation - everything works on 16+ simultaneous channels of MIDI, and the MIDI connector itself has been replaced by USB . It has been that way for a LONG time.
Guitarists have had very little use for MIDI (to the point where each new generation of MIDI guitar gear tends to do LESS with MIDI).
Since there is pretty much no demand, used MIDI Merge/Split boxes and MIDI patch bays are DIRT CHEAP. Look at eBay, and at used gear in stores. They really never "break" - there's not much to them.