Because you really aren't a tenor, or you don't have the skill or technique to sing the songs you want to sing. You should actually be able to sing much higher than a F5 if you are going to sing tenor songs for musical theatre, much less arias from operas or oratorios. You are looking at an ability to sing at least an A5. (We are talking about when the tenor music is written on the treble staff, but sung an octave below the soprano pitch?) There may be also problems with transitioning through vocal registers (see "passagio").
If you have a desire to sing actual arias from that are from real operas and classical oratorios (not pop songs sung in Italian by Il Divo or Andrea Bocelli)--you must have formal voice training. A lot of it.
From an actual teacher, not a do-it-yourself course. Bocelli as mediocre as he is, still has had lots of vocal training to even be as mediocre as he is.
Opera, real opera, is hard. Your range in opera is more of a baritone, but as I point out range alone does not determine voice type. You are still young, so your voice also has some maturing to do though it's past its initial pubescent change. Please ask your music or choir teacher for recommendations for voice teachers before you ruin what voice you have.
http://www.voiceteacher.com/male_voice.html (I'm not recommending the CDs, but this is a great informational site on singing and singing problems--I take voice lessons from local teachers)