This is because of the conversion of liturgical mixed metaphors, if you will–a classic case of where liturgical forms and popular devotions become entangled.
The vigil before the Blessed Sacrament was initially intended to commemorate the Our Lord’s vigil in Gethsemane, before His passion. This began on Maundy Thursday evening, and not knowing the exact hour at which His arrest took place, it became customary to keep the watch throughout the night into the morning of Good Friday, when the Mass of the Presantcified (a somewhat inaccurate term for the Liturgy of the Passion) took place. This in turn gave rise to various customs in different parts of Europe, including of visiting seven different churches during the night, IHO of the seven last words.
Though in early centuries the Blessed Sacrament was received by all who wished on Good Friday, by the Middle Ages the custom became for the priest alone to receive. The Most Holy was reserved only as viaticum until the Easter Vigil the next morning. Gradually the practice arose of setting up a sepulchre and keeping watch with Our Lord until the start of the Easter Vigil liturgy on Holy Saturtday morning, and the Sacrament reserved as viaticum was drawn into this. But the Good Friday Vigil at the tomb was a private devotion, not a public adoration, as at least the initial part of the vigil at the altar of repose was.
The result of this is a blurring of the garden and tomb vigils. When the Holy Week rites were revised in the 1950s, there was an emphasis on restoring ceremonies to their original and proper times, a trend that continued when the missal was revised in 1969-70. Part of this emphasis was the suppression of all-night vigils on both Thursday and Friday. The 1955 rubrics ordered that the adoration cease at mdinight on Maundy Thursday (though I know of a few old ethnic parishes that continued the all-night adoration, presumably with permission).