Why not? Because the IRS says it doesn't; the IRS says the money is subject to double taxation. If it did add to your basis you wouldn't be paying taxes twice on this money.
From Treasury Decision 8357, IRS 1991-2 C.B. 181:
If an individual defers more than the maximum for a taxable year under plans of two or more unrelated employers, generally none of the plans is disqualified due to the excess. The individual may not, however, exclude the excess deferrals from gross income. Also, unless the excess deferrals are timely corrected under section 402(g)(2), they are taxed again when distributed from the plan, since they do not increase the employee’s investment in the contract under section 72.
