Question

Thousands of bills introduced in congress are never voted on why does this happen

Answer 1

The reason for this one is because the Congress doesn't have much time to deal with bills that they think is not good for the people.  All the member in the Congress can introduce bills, they undergo the legal process to be read and scheduled for the hearing. 

Answer 2

Because they "die" in committee
After the bills introduced in Congress, the Congress has to vote on it and decide whether the bill should be passed.
Before the bill got passed, the Congress has to put several things as consideration, such as the budget availability or whether the bill could be considered as constitutional.