Under Spanish Constitution, supreme law in Spain, the right to determine the territory of Spain belongs to all Spaniards (article 2).
What some people claim as "democracy" or "the right to vote" is actually removing this right from the rest of Spaniards.
Aside from other claims and whether there are political or historical reasons, the voting has been declared illegal by the "Tribunal Constitucional", highest court in Spain. End of story, it's illegal, period. And Spain (mostly judges and police forces) are trying to defend the law.
Now a quick note on the alleged economical facts: no, Catalonia does not contribute 16 billion to the rest of the country, it's actually 2,4 billion (this is what Catalonia published last year). This is called "solidarity", and it is what contributes to other regions in Spain to grow and benefit the country as a whole. Madrid is also a significant net contributor. On the other hand, Spain holds 55 billion in debt from Catalonia, which is unable to finance by itself on international debt markets. So numbers don't add that quickly as it has been said here.