std parallel algorithm seems to use only 1 thread

I am using C++17 parallel standard library algorithms with the std::execution::par execution policy. I am using Ubuntu on a laptop with 4 cores, clang 11 compiler and cmake extension for VS Code for the build (although I also checked with a simple single command line compilation without using cmake).

Based on the following observations, it seems the program only uses 1 thread:

1. Run time is the same as with using std::execution::seq (regular, sequential algorithm)
2. Using top -H I see only 1 thread with ~100% cpu usage
3. Using Ubuntu's system monitor I see one core active during execution (but the active core may change between different calls to sort if I do repeats using a for loop).

Code example:

#include <vector>
#include <iostream>
#include <algorithm>
#include <execution>
#include <chrono>
#include <thread>

int main()
{
    const int N = 10000000;
    std::vector<int> vec(N);
    std::chrono::duration<double> elapsed;
    unsigned int nThreads = std::thread::hardware_concurrency();
    std::cout << "number of available threads: " << nThreads << "\n"; // this prints "4"

    auto tstart = std::chrono::high_resolution_clock::now();
    std::generate(vec.begin(), vec.end(), []() {return rand() % 100;});

    //std::sort(std::execution::seq, vec.begin(), vec.end());
    std::sort(std::execution::par, vec.begin(), vec.end());
    auto tfinish = std::chrono::high_resolution_clock::now();

    elapsed = tfinish - tstart;
    std::cout << "Elapsed time: " << elapsed.count() << std::endl;

    return 0;
}

I thought that maybe the problem was that I didn't tell cmake to link to pthread library. So I changed CMakeLists.txt:

project(my_proj LANGUAGES C CXX)
find_package (Threads)
target_link_libraries (my_proj ${CMAKE_THREAD_LIBS_INIT})

But it didn't make any change.

Why it doesn't seem to run in parallel?