Why do I feel that the brush strokes of some famous calligraphers are not as good as those written on the ground with a brush dipped in water?

I think what you said is very reasonable. It is easier to write on the ground with a brush dipped in water, because the hard brush on the floor feels soft, has a wide field of vision and is easy to control. For the famous writer's calligraphy, if you let him write anonymously to others, you will get a lot of bad reviews, and you may say that your calligraphy is better than his. Many famous calligraphers in China flatter each other, but there are exceptions, such as Huai Su, Fei Xinwo, Guo Moruo and Mao Zedong.