How could I generate .srt files only?

[otakutyrant]

I do not want those vtt, txt, json, and so on.

[ryanheise]

Using the -h option shows the help screen, and reveals this helpful part:

  --output_format {txt,vtt,srt,tsv,json,all}, -f {txt,vtt,srt,tsv,json,all}
                        format of the output file; if not specified, all
                        available formats will be produced (default: all)

So you can generate .srt files only by using the option --output_format srt.