(9) Let him sell me the cave of Machpelah that he owns, which is at the edge of his land. Let him sell it to me, at the full price, for a burial site in your midst.”

19)And then Abraham buried his wife Sarah in the cave of the field of Machpelah, facing Mamre—now Hebron—in the land of Canaan.

...Adam said: Whilst I am yet alive I will build for myself a mausoleum to rest therein. Adam said: If in the case of the tablets (of stone), just because in the future they will be written by the finger (of God), the waters of the Jordan are destined to flee before them; how much more so will this be the case with my body which His two hands kneaded, and because He breathed into my nostrils the breath of the spirit of His mouth? After my death they will come and take my bones, and they will make them into an image for idolatry; but verily I will put my coffin deep down beneath the cave and within the cave.

(5) So Moses the servant of the LORD died there, in the land of Moab, at the command of the LORD. (6) He buried him in the valley in the land of Moab, near Beth-peor; and no one knows his burial place to this day.

עד היום הזה
“until this day”
The principal reason for this is to make it impossible for anyone to be buried next to him. This was in order to prevent necromancers from using this site for their purposes.

...so that his place of burial should not be known to anyone. And Hashem did this since if his burial place were known, perhaps someone in future generations might err and turn him into a god.

(1) Ye shall make you no idols, neither shall ye rear you up a graven image, or a pillar, neither shall ye place any figured stone in your land, to bow down unto it; for I am the LORD your God.

(19) And Rachel died, and was buried in the way to Ephrath—the same is Beth-lehem. (20) And Jacob set up a pillar upon her grave; the same is the pillar of Rachel’s grave unto this day.

(18) And Jacob rose up early in the morning, and took the stone that he had put under his head, and set it up for a pillar, and poured oil upon the top of it. (20) And Jacob vowed a vow, saying: ‘If God will be with me, and will keep me in this way that I go, and will give me bread to eat, and raiment to put on, (21) so that I come back to my father’s house in peace, then shall the LORD be my God, (22) and this stone, which I have set up for a pillar, shall be God’s house; and of all that Thou shalt give me I will surely give the tenth unto Thee.’

(10) Let no one be found among you who consigns his son or daughter to the fire, or who is an augur, a soothsayer, a diviner, a sorcerer, (11) one who casts spells, or one who consults ghosts or familiar spirits, or one who inquires of the dead.

Rabban Shimon ben Gamliel says, "We do not make grave markers for tzadikim [righteous people]. Their words are their memorial."

(4) One should not visit the graves.

For a cemetery is the resting place of tzadikim (righteous people), and prayers are better accepted there. But don't aim your intentions towards the dead, but rather request of the Holy One, May He be Blessed, to have mercy on you in the merit of the tzadikim in the earth there.And encircle the cemetery, and give tzedakah (charity) before you make your pleas. And do not go to a grave twice in a single day.

(3) One may not walk on the cemetery or within four cubits of a corpse or a grave while [holding] a Torah scroll in his arm and read in it, [nor] may one recite the Tefillah [there]. Likewise, is it forbidden to recite [Biblical verses] orally [on the cemetery], unless it is for the honour of the dead as has been explained.

(1) Burial grounds must not be treated irreverently, — e.g., to ease oneself thereinor to eat or drink therein;and one does not read [Bible] nor study [Mishna] therein...