The following is an excerpt from the Artscroll Stone Chumash regarding Gen
25: "As is customary in the Torah, when a person's role in the development
of the narritive is completed, his life is summed up, even though he may
have lived many more years. Once Abraham, at the age of 140, had arranged
for the marriage of Isaac, the destiny of the Jewish people moved on to the
next generation, even though Abraham lived to the age of 175... All of this
is yet another illustration that the Torah is not a history book; if it
were, there would surely be more events of interest that might be recorded.
But since it is the story of the development of G-d's Chosen People, any
events tangential to this primary theme are extraneous."
Condie