Quarks are never observed on their own, a phenomenon known as "quark confinement," due to a fundamental property of the strong nuclear force, which is one of the fundamental forces of nature. The strong nuclear force binds quarks together to form composite particles called hadrons, such as protons and neutrons.
The strong nuclear force is mediated by particles called gluons, which interact with quarks. Unlike forces such as electromagnetism, which decrease with distance, the strong force actually increases with separation, making it more difficult to pull quarks apart. As you try to separate two quarks, the energy stored in the gluon field between them increases. At a certain point, it becomes energetically favorable to create a new quark-antiquark pair from the vacuum rather than continuing to separate the original quarks.
This process, known as quark pair production, results in the formation of additional quarks and antiquarks, forming color-neutral hadrons. The new quarks combine with the original quarks to create composite particles that are observed in experiments. This confinement of quarks within composite particles is a fundamental feature of the strong nuclear force and is a consequence of its unique nature.
It is important to note that the confinement of quarks has been extensively studied and supported by a wide range of experimental evidence and theoretical calculations. However, while the phenomenon of quark confinement is well-established, a complete analytical solution for the behavior of the strong force in all regimes is still an active area of research in theoretical physics.