It wasn’t always so, at least not as we understand it. In discussing Leo the Great, Hughes Oliphant Old’s The Reading and Preaching of the Scriptures in the Worship of the Christian Church, Vol. 2: The Patristic Age has this to say:
Lent has usually been explained as a period of forty days during which those preparing for baptism at Easter were instructed in the essentials of the faith and, along with the rest of the Church, observed a more intense discipline of prayer and fasting. That those preparing for the sacrament should submit themselves to the disciplines of repentance had been an essential dimension from the beginning. Repentance had been at the heart of the ministry of John the Baptist, and for the ancient Church the baptism of Jesus by John had been the prototype of Christian baptism. While the penitential disciplines of Lent had been directed primarily at catechumens it had been considered appropriate from earliest times that the whole Church join the catechumens in these disciplines. As early as the Didache this principle is clearly expressed. It was a way of supporting the catechumens in these disciplines and of extending to them the bond of fellowship.
It must have been about Leo’s time that the baptism of adults in Rome became more and more infrequent and the baptism of infants became more regular. With this transition to infant baptism the penitential disciplines of Lent would have lost their significance had not the threat of barbarian invasion, at the same time, imbued them with new importance. After all, one could hardly ask infants to fast for forty days. On the other hand, Christians saw in the threat of barbarian invasion a reason to intensify their self-examination and the disciplines of penitential prayer. Surely the importance of penitential preaching in Leo’s pulpit ministry is obvious; in face, it is rather heavily weighted in that direction, which the troubled days in which he preached surely help explain. It was in these days that the liturgical calendar began to develop and take its place as the primary principle for organising worship. As the Dark Ages progressed, Lent soon became the most important liturgical season of all. From very early in its development the liturgical calendar was stamped with a penitential case.
Lent, as liturgical churches generally understand it, was the product of a) the shift in the mode of baptism and b) the collapse of the civilisation.