Because thousands of authors would appaer on a research paper in our field.

For example, the famous paper from ATLAS reporting the observation of the Higgs boson has around 3000 authors. This is not unique. Because every paper from the ATLAS collaboration would have around 3000 authors. It is just a convention. All people who has direct and indirect contributions are authors.

It is not a field for heroes. We don't have Newton or Einstein in this field. As long as you work hard, you gain experience and become an expert in a limited research space.

I didn't know this fact before I entered this field. Now I am still in this field because I have to exist and support a family with two kids. It is difficult, especially for me. I have many wonderful ideas, but I don't have the freedom to realize or verify them. 

Therefore I have been writing "single-author" papers since I got my PhD. I promised to myself to write one paper per year. On the one hand, I feel satisfied to realize/verify my ideas using Monte Carlo simulations given that I cannot touch real data. On the other hand, I can enjoy the moment that the papers with only my name were cited by other people's works. 

Last year is tough for me. I stayed at home and was seeking for a job, but it is very unpleasant. The only financial support is from my wife. I have two kids. One is 5.5-year-old and the other 2.5-year-old. My wife's salary is not high. Anyway that time is very difficult. However, I kept finding new ideas and found two ideas were worth publishing. I wrote two papers. One was submitted to EPJC, and was rejected (the reviewer said my result was not surprising). The editor transfered my manuscript to EPJP and was rejected again due to the same reason. The other paper was submitted to JHEP. The reviewer made critical comments, which made me almost give up.  But I managed to improve according to the comments. I almost rewrote the paper. I apprecited the reviewer's comment because they were really helpful. Yesterday, this paper was accepted finally.

When I wrote single-author papers, I intentionally submitted them to different journals. Now the list is PLB, CPC, JPG, NIMA and JHEP. For sure, I will try my best to extend the list one item per year to keep my brain open for new ideas.