because

Numbers 8:16, 17
For the Levites have been wholly given to Me from among the sons of Israel. I have taken them for Myself in place of all who come first from the womb, the firstborn of all the sons of Israel. . . .

Numbers 18:15
The firstborn of every womb, whether man or beast, that is offered to the LORD belongs to you. But you must surely redeem every firstborn son and every firstborn male of unclean animals.

Exodus 13:2, 12
"Consecrate to Me every firstborn male. The firstborn from every womb among the Israelites belongs to Me, both of man and beast." . . .

Exodus 22:29
You must not hold back offerings from your granaries or vats. You are to give Me the firstborn of your sons.

Exodus 34:19
The first offspring of every womb belongs to Me, including all the firstborn males among your livestock, whether cattle or sheep.

Leviticus 27:26
But no one may consecrate a firstborn of the livestock, because a firstborn belongs to the LORD. Whether an animal from the herd or flock, it is the LORD's.

Ezekiel 44:30
The best of all the firstfruits and of every contribution from all your offerings will belong to the priests. You are to give your first batch of dough to the priest, so that a blessing may rest upon your homes.

Luke 2:23
(as it is written in the Law of the Lord: "Every firstborn male shall be consecrated to the Lord"),

Hebrews 12:23
in joyful assembly, to the congregation of the firstborn, enrolled in heaven. You have come to God the judge of all men, to the spirits of the righteous made perfect,

on the day

Exodus 12:29, 30
Now at midnight the LORD struck down every firstborn male in the land of Egypt, from the firstborn of Pharaoh, who sat on his throne, to the firstborn of the prisoner in the dungeon, as well as all the firstborn among the livestock. . . .

Exodus 13:15
And when Pharaoh stubbornly refused to let us go, the LORD killed every firstborn in the land of Egypt, both of man and beast. This is why I sacrifice to the LORD the firstborn male of every womb, but I redeem all the firstborn of my sons.'