Why a referendum instead of just declaring independence?

Because a declaration is words, and a referendum is a mandate. Independence has to be legitimate to last, and legitimacy in the modern world comes from the consent of the governed, expressed in a free and fair vote. A vote is what turns a wish into a nation the world will recognize.

Consent is what makes it real

Self-government means the people govern themselves. You cannot claim that and then hand a small group the power to declare a new country in everyone else's name. Every Western-style democracy that has won its independence in the modern era has let the people have the final say. Texas independence is built on the consent of the governed, full stop, and consent is something you demonstrate at the ballot box, not something a leader announces from a podium.

Recognition runs on legitimacy

A new nation lives or dies on whether other governments deal with it as a nation. The single most important thing a process can produce is recognition, and recognition follows legitimacy. A peaceful, lawful referendum, conducted under the laws of Texas with millions of Texans voting, is about as legitimate as a founding act gets. It makes it far harder for Washington or anyone else to dismiss the result, and far easier for trading partners to formalize relations. A bare declaration, with no vote behind it, hands opponents the easiest possible argument: that it does not speak for the people.

It keeps the path peaceful and legal

The referendum is also what keeps this lawful and bloodless. A vote is plainly not an act of force. It is not levying war and it is not a rebellion. It is the most ordinary instrument of democracy there is. Routing independence through a vote keeps it squarely inside the law, which is exactly why the movement insists on it. The legal path must be followed, and the legal path is the ballot.

It is the difference between 1861 and now

History makes the point. The 1861 secession was carried out by conventions, not by a free vote of the whole people, and it is rightly remembered as illegitimate. The lesson Texans took from that is not to repeat it. Today's path is the opposite: every eligible Texan, of every background, deciding together in the open. The referendum is what separates a legitimate act of self-determination from a power grab.

The bottom line

A declaration alone is fragile and contestable. A referendum is durable and undeniable. We do it by the vote because the vote is what makes Texas independence legitimate, recognized, and permanent.