The reason:

The reason Hashem requires us to perform effort in the physical realm is because the blessing must come through the nature of the world. Just as all Divine blessing that enters the world does so through the laws of nature, so too one must create a vessel within nature for G-d’s blessing.[1] If one does not make a vessel through nature, and relies on a miracle, the miracle may not occur and if it does it will diminish his merits.[2] [In other words, the concept of Histadlus is rooted in the concept of “Ein Somchim Al Haeis[3].”[4]] The cause for the requirement of investing effort is due to the curse that G-d gave Adam that with the sweat of his brow he should eat bread, and ever since that point, we are required to place effort to make a living, until the days of the future when loaves will grow from the earth.[5]

[1] Likkutei Torah Parshas Ki Seitzei p. 37; Derech Mitzvosecha Mitzvas Tiglachas Metzora 3 p. 213 “The reason for this is because the divine energy that is drawn to this physical world needs to descend from one level to the next until it is invested within a thick garment in order so from it will come physical kindness and blessing in this world and it is on this the man is judged every day. This is indeed the reason why one must be involved in business, as the energy in this world is drawn down through the garments of nature. This means that it is invested and concealed within nature until it appears as if the world is run by the rules of nature even though everything in truth comes from God Himself and it is just that he conceals His Godliness to the point that miracles and unnatural occurrences are not apparent and revealed. Thus, when God gives sustenance to a Jew, he does not do so through miracle such as to cause bread and meat to descend from the heavens as was done with the generation of the Mun, as this is not natural, and rather he sends His blessing that the person should make a profit in this business which is within nature and by which the person can confuse himself and say that it is his own efforts that brought his profits.”; Igros Kodesh 2:179

[2] Igros Kodesh 2:179; See Ramban Shaar Hagemul; Shabbos 32a

[3] Shabbos 32a

[4] Igros Kodesh ibid

[5] Mesilas Yesharim ibid; Michtav Meliyahu Vol. 1 Inyanei Emuna Ubitachon