I had a side business. In my primary employment I made over $142,800,company and I stopped paying ss tax. On my side business, why still paying the employer side of ss?

At the end of the year, you will get the excess Social Security tax refunded. For tax year 2021, you'll have excess Social Security withholdings if the sum of multiple employers' withholdings exceeds $8,853.60 per taxpayer.

You don't need to take any action. TurboTax will automatically add the excess to your federal refund or subtract it from the federal taxes you owe, whichever applies.

The excess will appear as a tax credit on Line 11 of your Form 1040, Schedule 3.

If you have a schedule C business, your schedule SE will calculate the self-employment tax you owe and will take into account the tax you have already paid through your W-2.   If you are already above the social security max with your W-2 job, you won't pay social security tax on your side business.  You will still pay 2.9% for medicare.