Sure, you learn the most in your first 6 months by quantity but that doesn't mean you learn the most valuable things. You won't learn anything of depth in half a year.
Learning non-trivial codebase often takes much more time. I worked on bigger projects and someone who worked there only six months would not be allowed to do bigger or core changes.
Then there is domain knowledge (finance, healthcare, law, etc) if you do that kind of software. Six months is enough to learn surface in anything non-trivial and everything is much more effective if developer already learned that.
Of course, last two points are not really valid for small projects world.
Agreed. It is the quickest way to become a 10x employee but unfortunately your current company won't give you a substantial raise for knowing their product and domain well and it is not transferable to other workplaces.
Most companies wont let you make any high risk or big impact decisions in your first year of employment with them (regardless of whether you get hired at a junior or senior or tech lead level).
Experience in solving the more difficult problems that require expertise with the companies particular stack and internal architecture.
And many more. There are ways to mitigate these problems but few do.
Kidding. You ask a legitimate question. This is by no means a comprehensive answer, but...
1. Domain knowledge. I think most of us enjoy the creative and problem solving parts of software engineering. That takes domain knowledge and domain knowledge takes time. You will get an intro to your domain in a year, but mastery? I would bet against it.
2. Working on a hard project from start to finish and sticking around for the lessons learned after. This is just a math problem. You're useless for at least 30 days no matter how good you are. (More like 90 IMO...). So if you start working on a larger project with a new team, and that project takes 6 months, your year is ending rapidly.
3. Mastering truly large code bases. Not everything is a rails app. Some things are just hard. Mastery is difficult to achieve.
4. Engineering leadership. Even if you don't want to be on a management track, it's important for a seasoned, senior-level engineer to be able to lead/drive a project from start to finish.
...And I suppose you may think "But I do all of those." And maybe you do? Or maybe it's just really hard to see something that you're convinced isn't there.