Excuse me, is calligraphy written so that everyone can't recognize what it is, even if it is good calligraphy?

It doesn't mean you don't know that your handwriting is good. I don't know if I haven't learned cursive script. Other calligraphy is easy to recognize, such as Wang Xizhi's cursive script Preface to Lanting. One more thing, most people who practice calligraphy practice traditional Chinese characters, which may be one of the reasons if they have enough knowledge of Chinese characters.

Besides, calligraphy is like art, China calligraphy is an ancient art, from Oracle Bone Inscriptions, Bronze inscriptions to Da Zhuan, Xiao Zhuan and Li Shu, to cursive script, regular script and running script in the Eastern Han Dynasty and Wei and Jin Dynasties, calligraphy has always exuded artistic charm.

Just as you said, "Song Huizong and Yan Zhenqing don't have many words, and their handwriting is too sloppy", the main reason is that your cursive script is not up to standard, and everyone's writing method is different, and then the written words will be ever-changing, but as long as you practice hard, you will definitely make achievements in calligraphy art.