This is incorrect. People do make over a million, e.g. Sundar Pichai makes $2 million base. The reason the tax is a lower percentage for a millionaire is because the social security tax is capped. You only pay it on the first $184,000 (this increases yearly) that you earn.

It is important to note social security tax is basically forced retirement savings. In theory, you are funding your own retirement through it (the more you pay in, the more you get out later). This is very different than ordinary income tax, which is not capped (that’s not to say it’s high enough, or that there aren’t ways to hide, defer, or obscure income, and pay less than expected or reasonable).