Mesons are composite particles made up of a quark and an antiquark bound together by the strong nuclear force, which is mediated by the exchange of gluons. The strong force is unique among the fundamental forces in that it becomes stronger as the distance between two quarks increases, unlike the electromagnetic or gravitational forces.
The reason mesons cannot be separated into individual quarks is due to a phenomenon called color confinement. Quarks come in three "colors" (red, green, and blue), and antiquarks come in three corresponding "anticolors" (antired, antigreen, and antiblue). According to the theory of quantum chromodynamics (QCD), which describes the strong force, the total color charge of a particle must be colorless. In other words, the combination of quark and antiquark colors in a meson must add up to "white" or colorless.
When you try to separate the quark and antiquark in a meson, the energy required to do so increases with distance. As you pull them apart, the energy stored in the gluon field between them also increases, which in turn generates additional quark-antiquark pairs. This process continues until the energy is sufficient to produce a new quark-antiquark pair, resulting in the creation of new mesons. As a result, you always end up with multiple mesons rather than isolated quarks.
This phenomenon of color confinement has been observed experimentally and is a fundamental aspect of the strong force. Although quarks have been detected individually in high-energy collisions, they cannot exist freely in isolation due to the strong force and the principles of color confinement.