For example, if a state could secede from "the Union", e.g. the United States of America, with simply a majority vote in a referendum that could be called for at any time, how many states do you think would still be in the so-called United States of America? I am pretty sure that most of the Southern and many of the Western states would no longer be a part of our country if that was all it took.
To put a possibly useful spin on this, lets briefly review how the two different constitutions came about. The UK as a country came into existence over a period of more than a millennia. One of the benchmarks of this formation process was, of course, the famous Magna Carta of 1215. That document, and the events that precipitated it, is now 800 years old. Although there were certainly civil wars and revolutions in the region now called the United Kingdom, there was never a time when everything was brought together into a single document and put into writing. It was a much more organic process, incremental, and with a very long history.
These darn foreigners are always full of surprises.