Converting Midi data to notes is easy. A midi message consists of some routing information, telling the receiving midi device that this message is for it. Then comes a number to tell the device which note to play, and how loud, then another to tell it when to play and when to stop. There may be several others to cover vibrato etc.
But the point is, that every mid message contains numbers describing the pitch and duration of the note. It has no audio content- it's numbers not music, and not even digitised music, it's computer control codes.
That's why it's easy to show Midi as notation. To write music on the Staves, you need to know the pitch, duration and timing of each note. That is all represented by data in the midi message.
What comes out of your guitar is audio, whether it is air vibrations or an electronic signal.
Guitars do not play one note at a time- midi always does (even if the notes are too close together to tell the difference).
A midi converter for guitar has to detect and record the time, pitch and volume of every sound the guitar makes, decide which are music and which are not, and cope with the many inaccuracies caused by intonation faults, string bending, vibrato etc.. and it has to do this for each string separately, without allowing one to interfere with the next, and produce a result fast enough that you can't hear any latency or delay in response.
Apart from the dedicated midi pickup/processor set ups, none of which are quite perfect as yet, the only software I know that can cope with more than one note at a time is Melodyne Editor. That is like a much more sophisticated of the much abused Autotune, and unless you really need it it would probably be cheaper to hire someone who can take musical dictation.
The human brain is still far ahead of any silicon based software when it comes to knowing a B sharp when it hears one.
Cheers
Dave