I'm having trouble finding an answer that actually addresses the question asked, so here goes:
Marriage ceremonies are, with a few exceptions, affairs based on religious, not legal tradition. Religions tend to have various marital rulesets based on millenia-old problems establishing paternity, inheritance – and to a lesser extent, the preservation of the bride's rights and powers after producing offspring.
If you consider the historical origin of marriage itself as a sales process, the ceremony is basically the equivalent of a contract signing – and most contracts would be useless if they were written as "this holds true until you're no longer feelin' it". Rather, by making it clear that the arrangement was permanent (barring certain fairly common clauses), the gravity of the institution, and the alliances forged and lands distributed as a result could be viewed as much more stable and long-lasting than they otherwise would.
Today, 90% of the original reasons for inter-familial marriage are viewed as relatively unimportant (British royalty and Hollywood dream-couples aside), with love and affection being the popular selection principles. In turn, this makes marriage inherently more of a "couple" than a "families" affair. Romeo and Juliet can get married, and the Montague-Capulet hatefest is free to continue unabated.
Unfortunately, neither established religions, nor the law, have moved as quickly as the modern human heart – which creates the disparity at the core of the question:
On the one hand, the marriage ceremony is (usually) filled with language of permanence at all costs,
While on the other hand, most "functional" marriages are much more complex, and usually based on factors that are never even mentioned during the marriage ceremony.
So, really, it's simple: the reason most marriage ceremonies assume permanence is because the ceremonies are largely cut from old, outdated cloth. Whether or not this is a good thing is another question entirely.
This is not to say that it's impossible to create a ceremony of that is both legal, and more self-aware/modernized to reflect the specific desires and agreements of a given couple. With the widespread availability of "retail prenups", more exotic vehicles like the "Relationship LLC", and the burgeoning availability of non-heterosexual marriage, it seems likely that the problem described in the question will be at least more significantly addressed in the next few decades.
For the record – my personal recommendation would be to subsume all marriage within existing adoption law – allowing for many additional forms of marital constructs in one fell swoop (non-heteronormative, non-monogamous, asynchronous, pure platonic, etc), as well as building in probationary periods, and routine checkins – not simply to police the newlywed, but to assist them in establishing relationships that truly can withstand the many tests of time. This might also make it easier for everyone involved to address the conclusion of a marriage in a less inherently negative/failure-oriented manner.
Why do marriage ceremonies assume marriage is forever, when for many, it isn't going to be?
(It… kinda… is… that easy. Courtesy of 
deano.com 