Most engineers don't get a PE because they don't necessarily need one in order to practice.
If you work in an engineering firm which has at least one person on staff with a PE, an engineer can be considered to be working under the direction of the PE.
If you want to call yourself an engineer, then state law will probably require that you be a PE. Same if you want to open an engineering firm.
One big reason a lot of engineers don't get the PE in the states is that licensure is on a state-by-state basis. If, for example, you get your PE in Iowa, and you decide to take a job in California, your PE from Iowa won't be recognized automatically in California. You can apply for what is known as reciprocity, where you ask the California licensing board to accept your Iowa credentials.
It used to be that if your original PE certification came from one of the bigger states, like New York or California, receiving reciprocity from the other states would largely be a formality, but going from a small state to a big state, recognition of your original credentials would not be automatic.
I believe that the NCEES has been trying to nationalize the scope of PE certification in the US, so that these 'Catch-22' situations would be eliminated.
Now, too, one can receive PE licensure in many different types of engineering. It used to be that the FE was split between Mechanical and Civil engineering. If you were neither of those, you had to do extra preparation for the test. Now, with other types of engineering recognized for PE status, there are different tests more closely tailored to the candidate engineer's educational background.
One interesting aside: I'm a naval architect, and naval architecture is one of the new disciplines which can receive PE certification in some states. However, building architects reportedly objected to NAs with PEs calling themselves naval 'architects', because they hadn't gone through the process the AIA uses to certify architects. You can still get a PE in NA, but you have to call yourself something besides a 'naval architect'.