When I look at the sky, the reflective surface of the ground looks nice, but when I look at the ground, it becomes blurry.
Because it a screen space reflection, without real ray tracing engine either need rerender with different projection or to sample color for reflection elsewhere, screen space reflection utilize the fact that some of reflected scene is already rendered on the screen and use it for reflection, if the things that is reflected is not on the screen (Sky in this case) screen space reflection have nothing to sample from and other reflection system is used (cubemap in this case which are lower resolution)
If what you have is not sufficient then you need to use planar reflection, which will rerender the scene with different projection needed to recreate the proper reflection form stuff that camera dont see
You can also set reflection probe to have better resolution:
Or disable screen space reflections so you dont have that effect