That's simply not how it's done for the self-employed. It has partly to do with the fact that the employer contributions reduce the amount from which the employer contribution is calculated. It also has to do with the fact that and employee elective deferral reduces the individual's taxable income, not the employer's taxable income, and that the employee elective deferral can affect the amount of employer contribution that is permitted, so the employee and employer contributions must be determined concurrently.
