That's fair criticism. The deeper reason why it's not a constitution is because it's not the cornerstone of legal, moral, and existential rights of the signato

That's fair criticism. The deeper reason why it's not a constitution is because it's not the cornerstone of legal, moral, and existential rights of the signatory constituent countries. These countries each have their own constitution that they are beholden to, with no higher authority to fall back to. The Treaty of Maastricht (and later, Lisbon) can't be guaranteed to have precedence when called upon if they conflict with the constituent countries' constitution(s), and all legislation covered under the treaties must be implemented separately and completely as locally applicable laws before going into effect. The EU regulations generally prescribe a deadline for implementing such regulations, but a country could (and sometimes does, see e.g. Hungary under Orban) break its obligations under the Treaty and ignore or never implement the agreed-upon regulations, and the EU would be unable to do much to enforce its regulations.