The inability to isolate a quark, a phenomenon known as color confinement, is a fundamental property of the strong nuclear force, which governs the behavior of quarks. Color confinement is a unique feature of the theory of quantum chromodynamics (QCD), which describes the strong interaction between quarks and gluons.
Quarks carry a property called "color charge," which is analogous to electric charge in electromagnetism. However, unlike electric charge, which comes in positive and negative values, color charge comes in three different types: red, green, and blue (these are just labels and have no relation to actual colors). Antiquarks carry corresponding anti-color charges (antired, antigreen, and antiblue).
The strong force, mediated by gluons, acts to exchange color charges between quarks. As quarks move apart, the energy stored in the color field between them increases. This energy is proportional to the distance between the quarks, and as the distance increases, the energy becomes large enough to create new quark-antiquark pairs. Instead of isolating a single quark, the energy is used to create a new quark-antiquark pair, resulting in the formation of color-neutral particles called hadrons.
In other words, when we try to pull a quark away from other quarks, the energy required to separate them increases. Eventually, there is enough energy to create a new quark-antiquark pair from the vacuum, resulting in the formation of new hadrons. This prevents the isolation of individual quarks.
Experimental observations have confirmed this phenomenon. No experiment has been able to separate and detect individual quarks in isolation. Quarks can only exist within the composite particles (hadrons) that they form due to color confinement.