I have a hard time understanding. At least for what I've seen/done, it would take about a year of experience to be of any real value. I'd say you start seeing real dividends from an employee near the three year mark. I think the primary exception would be where you have a big gaping hole in an organization...like building a data science program or something from the ground up. But if you've got software and customers long long past the 1.0 stage to support, and someone is going to (someday) understand/contribute to the core? Think about Google's monolith or the Linux kernel...sure most software isn't that mature/grand, but there are many projects out there that are closer to that than greenfield. And it's not just the code, it's the developed relationships/rhythm with coworkers and customers.
Maybe I've explained it to myself, I don't know. The difference may be startup companies or projects versus mature ones. At least in the latter case, it seems to me that if a company retains an engineer for less than 2-3 years, they've almost certainly lost on that investment.