As some others have pointed out, paying social security directly costs companies money.
However, there is also a secondary set of costs to social security. Social security dramatically decreases a companies grip on a worker as the worker grows closer to retirement. A lot of workers would not be able to retire with out the income that social security provides. Without social security, once a worker passes the age at which they become less desirable for other companies to hire away then they essentially become slaves. And knowing you’re heading towards what is essentially slavery is going to have a lot of knock-on effects too. Pay and compensation negotiations, changing companies, how afraid people will be of being fired, ect… Are all going to swing heavily into the companies favor.