Quite simply, in order for a marriage to be consummated, it must first exist. Sexual activity between two unmarried people does not somehow "carry over" after they marry each other. Their activity is the sin of fornication:
Fornication is carnal union between an unmarried man and an unmarried woman. It is gravely contrary to the dignity of persons and of human sexuality, which is naturally ordered to the good of spouses and the generation and education of children.
(Catechism of the Catholic Church paragraph 2353)
A sinful activity has no relation to an activity which is not only not a sin, but part of the purpose of a state ordained by God.
The difference is that marriage involves a total giving of each spouse to each:
"Conjugal love involves a totality, in which all the elements of the person enter—appeal of the body and instinct, power of feeling and affectivity, aspiration of the spirit and of will. It aims at a deeply personal unity, a unity that, beyond union in one flesh, leads to forming one heart and soul; it demands indissolubility and faithfulness in definitive mutual giving; and it is open to fertility."
(Catechism paragraph 1643. The quote is from Pope St. John Paul II's apostolic exhortation "Familiaris Consortio".)
Sex without marriage is not the full mutual giving that marriage demands; it's a different kind of activity than sex in marriage—no matter what the intentions of the participants are at the time. Thus, only sexual activity undertaken after marriage can be considered a consummation of the marriage, and then only a certain kind:
A valid marriage between the baptized is called ratum tantum if it has not been consummated: it is called ratum et consummatum if the spouses have performed between themselves in a human fashion a conjugal act which is suitable in itself for the procreation of offspring.... by which the spouses become one flesh.
(Code of Canon Law, Canon 1061 section 1)