Most pensions, 401(k) plans, etc. are in addition to social security. They are not intended to replace social security and employees covered by these plans are also earning social security credits. When these plans are equitably distributed between the spouses and the spouses are left with their statutory claim, or no claim, for social security no unfairness results.
But federal retirement plans and many state government plans are not intended to supplement social security, they are intended to provide a retirement income without social security. Both the employee deduction and employer contributions are higher, no social security taxes are paid and the federal and state employees are not earning social security credits.