It’s because the concept of a particle having definite properties like position and momentum doesn’t hold in the quantum world. Until a measurement is made, the particle is in a superposition of all possible states but with different probabilities, these are described by its wavefunction, which encodes what the various particle variables (position, spin, momentum, etc.) could be.

So, it’s not a measurement issue that introduces the uncertainty; it’s already there as a fundamental property of the particle’s quantum state.

Measurements merely “choose” one of the many possible outcomes, collapsing the wavefunction and in turn making exact measurement of other complementary properties impossible (because the mere act of measuring one variable causes the system to transition into a new state with its own set of probabilities and uncertainties for all variables)

And because these are inherent limitations dictated by quantum mechanics and the uncertainty principle, even if we could know the current state of every particle in the universe, we still couldn’t accurately predict the future because of that fundamental uncertainty.