They do, but in many states it goes towards your minimum wage. https://en.wikipedia.org/wiki/Tipped_wage

The most extreme example is in a state like Alabama where the minimum wage is $7.25 and the minimum tipped wage is $2.13 (these are the federal minimums):

   +------------+----------+----------+
   |Tips        |Wage      |Final     |
   |            |          |Wage      |
   +------------+----------+----------+
   |0           |7.25      |7.25      |
   +------------+----------+----------+
   |<5.12       |7.25-$tips|7.25      |
   |            |          |          |
   +------------+----------+----------+
   |>=5.12      |2.13      |2.13+$tips|
   |            |          |          |
   +------------+----------+----------+

So your first 5.12 (=7.25-2.13) in tips are worth nothing. Of course, you can eventually earn more than the minimum wage, in which case the tips aren't exactly worth nothing, but they are worth the value of pushing your final wage above this threshold.