Serial MIDI and Arduino MIDI

The default MIDI instance is on Serial, it's not USB MIDI.

You have to use this:

MIDI_CREATE_INSTANCE(UsbTransport, sUsbTransport, MIDI);

Have a look at the examples:
https://github.com/FortySevenEffects/arduino_midi_library/blob/master/examples/MidiUSB/MidiUSB.ino

Pieter