Everyone pays into Medicare and social security, but not everyone pays a federal income tax.
You must have forgotten about this Romney clip.
If you are single and make less than somewhere around $13000/year, you pay no federal income taxes. It’s in the upper 20000s for married. So, 47% of Americans fall into that.
SS is 6.2% of your income. Medicare is in the mid 1%. Something like 1.6%. There is a cap though. So, if you make over 400,000, you’d pay less than 6.2%.