Why is the last semiquaver outside the last triplet of the measure?
Because it should be!
You can think of the last quarter beat of the measure as being divided into three voices:
a) An eighth rest and two eighth notes (
b) A dotted eighth note and a sixteenth note (
c) A quarter note (
The last eighth note of voice a) should be placed approximately one third from the end of an imagined full quarter note length, while the sixteenth note of voice b) should be placed one quarter from the end of this imagined length.
The last semiquaver isn't going to be played, right?
Wrong! Voice b) is the melody! The other notes are (mainly) accompaniment.2
Listen to this recording and you will understand how it should be played. The sheet music or notation used for that recording might not be identical to yours, but you should get the idea.
The music, "Piano Sonata No. 14" (in C sharp) by Ludwig van Beethoven, is a piano sonata that in your sheet music has been adapted for the guitar. It is nicknamed the Moonlight Sonata, (or in French Sonate au clair de lune).
1The notes sounding on the guitar is actually one octave down, but I chose here to write the octave numberings generally associated with the lines of the staff.
2This is a slight overstatement in order to emphasize the importance of voice b).