Two reasons:

One, even if you believe the theory to be correct they don’t actually exist on their own. They cannot be isolated. They are an attempt at explaining the traces found in particle accelerator after collisions.

Two, if you look at the history of quarks (for lack of a better term) you’ll find that the theory was constantly challenged over time and changed to fit the new findings. The definition of quarks keep changing and will keep changing until they are replaced by true building blocks.

35 MeV

Particle science is not immune to political corruption. Careers are at stake