Why don't we have any more Apostles?

I think that when most Christians refer an "apostle" they are thinking of the Twelve called by Jesus or maybe Paul and James. Many believe that they were a gift for the early Church but now there are no apostles. 

Apostle simply means "one who is sent." Paul introduces himself as an apostle on many occasions in the introductions of his epistles (Romans 1:1, 1Corinthians 1:1, etc). 

"Paul, a servant of Christ Jesus, called to be an apostle ..." Romans 1:1

When was Paul called? 

"But the Lord said to him, 'Go, for he is a chosen instrument of mine to carry my name before the Gentiles and kings and the sons of Israel.' " Acts 9:15 

"While they were worshiping the Lord and fasting, the Holy Spirit said, 'Set apart for me Barnabas and Saul for the work to which I have called them.' Then after fasting and praying they laid their hands on them and sent them off." Acts 13:2 

 It sounds like Paul calls himself an apostle because he was sent out of proclaim the gospel. So is there a real difference between an apostle and a missionary? Doesn't look like it to me. So then why do we say missionary instead of apostle? Missionary is not a word found in the bible.