Because
Numbers 8:16
For they are wholly given unto me from among the children of Israel; instead of such as open every womb, even instead of the firstborn of all the children of Israel, have I taken them unto me.
Numbers 8:17
For all the firstborn of the children of Israel are mine, both man and beast: on the day that I smote every firstborn in the land of Egypt I sanctified them for myself.
Numbers 18:15
Every thing that openeth the matrix in all flesh, which they bring unto the LORD, whether it be of men or beasts, shall be thine: nevertheless the firstborn of man shalt thou surely redeem, and the firstling of unclean beasts shalt thou redeem.
Exodus 13:2
Sanctify unto me all the firstborn, whatsoever openeth the womb among the children of Israel, both of man and of beast: it is mine.
Exodus 13:12
That thou shalt set apart unto the LORD all that openeth the matrix, and every firstling that cometh of a beast which thou hast; the males shall be the LORD'S.
Exodus 22:29
Thou shalt not delay to offer the first of thy ripe fruits, and of thy liquors: the firstborn of thy sons shalt thou give unto me.
Exodus 34:19
All that openeth the matrix is mine; and every firstling among thy cattle, whether ox or sheep, that is male.
Leviticus 27:26
Only the firstling of the beasts, which should be the LORD'S firstling, no man shall sanctify it; whether it be ox, or sheep: it is the LORD'S.
Ezekiel 44:30
And the first of all the firstfruits of all things, and every oblation of all, of every sort of your oblations, shall be the priest's: ye shall also give unto the priest the first of your dough, that he may cause the blessing to rest in thine house.
Luke 2:23
As it is written in the law of the Lord, Every male that openeth the womb shall be called holy to the Lord;)
Hebrews 12:23
To the general assembly and church of the firstborn, which are written in heaven, and to God the Judge of all, and to the spirits of just men made perfect,
on the day
Exodus 12:29
And it came to pass, that at midnight the LORD smote all the firstborn in the land of Egypt, from the firstborn of Pharaoh that sat on his throne unto the firstborn of the captive that was in the dungeon; and all the firstborn of cattle.
Exodus 12:30
And Pharaoh rose up in the night, he, and all his servants, and all the Egyptians; and there was a great cry in Egypt; for there was not a house where there was not one dead.
Exodus 13:15
And it came to pass, when Pharaoh would hardly let us go, that the LORD slew all the firstborn in the land of Egypt, both the firstborn of man, and the firstborn of beast: therefore I sacrifice to the LORD all that openeth the matrix, being males; but all the firstborn of my children I redeem.