While this might sound like a good question on the surface, it does not get to the "root" of the problem. Let me explain ...

Yes, most companies prefer to hire people who already have the skills & experience rather than train "junior". This is not because companies don't want to develop the skills of their employees; it's simple economics. The biggest bottleneck in any company is experienced people. The senior engineers who already understand all the systems, have been to all the product meetings and solved many critical bugs in production.
These people are the "goose that lays the golden egg". Most companies are looking for more of these "golden geese" who can be effective & contribute to the product immediately because the "ROI" on these people is 10x (or more!).

Training someone up from scratch in a key tech and all the companies systems usually has a negative "ROI" for the first 1-6 months and distracts senior people so it's a "lose-lose" in the short-run! Add to the fact that most companies have a "LIFO" pattern with hiring (the most recent hires are usually the people who exist first!), and many hiring managers (HR) are put off by the idea of hiring people who do not already have the required skills.

Consider the following often repeated quote/saying:

CFO: What happens if we train them and they leave?
CEO: What happens if we don’t and they stay?

A lot of people have the mindset that training people costs too much time, money & effort and it distracts the key people in the company away from their focus (building the product).

This is not the fault of the company or the people working there. It's a "systems problem"; most companies simply don't have an effective system for "on-boarding & training" new people.

I've worked for several companies over the past 20 years (including starting my own twice) and have been responsible for hiring & training thousands of people.

Training people in tech skills, company culture & workflow simultaneously is a "hard problem". If you can get a "head start" on at least one of these areas the chance of successfully integrating someone is much higher. HR people know this so they want to "check" as many of the skills boxes as possible up-front. You as the "junior dev" can use this information to your advantage and invest a few hours up-front to demonstrate the necessary skills and make the HR/hiring manager's job much easier!

My advice to any "junior" person reading this:

1. Focus on your own learning/skills for at least an hour every day (preferably first thing in the morning).

2. Share your learning somewhere public e.g: GitHub or a Blog. that way the hiring manager reviewing your "CV" has a clear indication that you are "fast learner" and a "team player" who shares what they learn to help others "level up".

3. Pick the skill/tech/tool that is most valuable in your chosen industry/sector or even target it to a specific company you want to work for. e.g: if you know that AirBnB uses React.js https://stackshare.io/airbnb/airbnb you find and devour all the best tutorials for learning React.

4. Consolidate your learning into a tutorial of your own to show that you have understood the tech/tool.

5. Link to it directly from your CV/LinkedIn.

Seriously, this will take you 20h at most. You could get it done in a week and it will transform your "hireability" from "no thanks" to "when can you start?".

I know this because I have used this strategy to get jobs & contract work in the past to excellent effect. Investing in your skills and sharing your knowledge is the single best time-investment you can make. It's a 1000x ROI! Put in 20h of focussed effort and you will get an extra $200K in the next 2-5 years. Guaranteed.

My advice to any company wanting to solve the "problem" of hiring "junior" people and making them effective as fast as possible is:

1. Commit to becoming a "learning organisation" where everyone in the company shares as much of what they learn as possible.

2. Establish metrics for learning in your company! "What gets measured gets done". If there is no actively tracked & visible metric for each person's learning, people will stagnate and default to using their existing "hammer". https://en.wikipedia.org/wiki/Law_of_the_instrument what you want is people who are proactively learning new skills/tech/tools and then bring those skills into the company to improve effectiveness or build features that your current tech does not allow!

3. Systematically share anything that is not "sensitive" or "secret sauce" in public. Having private wikis with lots of learning is fine for internal use, but what if people could learn your "stack" before they join your company/team?

4. Hire the people who proactively contribute to the learning materials without being prompted. This is the mindset you are looking for: people who want to learn and share what they know regardless of getting paid.

Anyone looking for a proven example of any of this, see: https://github.com/dwyl?q=learn dwyl is a bootstrapped, profitable Open Source software company that shares all of it's learning in public. [disclaimer: I co-founded it!]