He joined last year May and left recently. About one year of stay. I wonder one year is enough time for programmers to understand codebase, let alone meaningfu

He joined last year May and left recently. About one year of stay. I wonder one year is enough time for programmers to understand codebase, let alone meaningfully contributing patches? But then we see that job hopping is increasing common, which results in the drop in product qualities. I wonder what values are the job hoppers adding to the company.