Because Christ himself outlawed divorce. Whereas God tolerated divorce in the Old Testament, due to man's degree of unenlightened-ness, once the Golden Rules were set down by Christ - thus enlightening mankind to "The Way, the Truth, and the Life" - there becomes no justification for divorce, no valid reason a spiritual/physical union should just be declared "dead" because someone stops "feeling" love or stops physically "demonstrating" love. If one spouse obstinantly presents a threat to the life and physical well-being of a family, the Church understands that a civil divorce may become necessary, however, no re-marriage may occur unless the original is annulled.
Annullment does not occur unless the Church decides that one or both of the spouses in question entered into the marriage contract under false pretenses, under duress, with psychological deformity, or with the foreknowledge of complete and total infertility (inability to consummate the union). Until and unless this occurs, every marriage is as unbreakable as Christ's bond with his Church.
As stated, true - Except we have many marriages that break up -for reasons that neither you nor I like - but they still break up and do not conform to the above criteria - and still some get an annulment. Not good and people think money talks.
Only about 5 to 10 percent of Catholic divorces get annulments. What do we do write them off? Many do not apply for annulment because they know they had a valid marriage and to seek an annulment would be a lie and a violation of their consciences. What pastoral procedure would you suggest? Face it -Good marriage sometimes die.