Because the numbers are not random or even sequential.
As has been posted numerous times, the first five relate to the location and month/year of issuance.
And numerous number combinations are not used (e.g., “000”).
Social Security Administration has assigned about 531 million Social Security numbers as of March 2023. There are still about 358 million numbers left to assign, and those are expected to last for approximately the next 70 years. The total available is 889 million.
In 2011, the SSA stopped allocating the so-called “area number” (the first three digits) based on address, and now allocates most “area numbers” that had gone unassigned. This would include all of those that start with 7 or 8.
The 900 - 999 series is still not used for SSNs; that series is used for “Taxpayer Identification Numbers”. They are given to individuals who aren’t eligible for Social Security numbers, including undocumented immigrants.
Before 2011, the middle two numbers were “group” numbers used for sorting, before the advent of computers. With computers, “grouping” is unnecessary. However, the “00” “group number” has never been issued, and regardless of the advent of computers, the SSA will continue the practice of excluding the zero-zero combination.
The first three numbers “000” were never used and here again, regardless of the advent of computers, the combination of zero-zero-zero at the beginning will still not be used. Likewise the last four numbers “0000” were never used and will still not be used past 2011.
https://www.marketplace.org/2023/03/10/will-we-ever-run-out-of-social-security-numbers/