Open source can be a wonderful thing when people come together and build something cool. Unfortunately, maintainer burnout has been a thing recently and part of the reason is that IT and open source are seen as a way to get a lucrative job. Some (and by no means all) people disregard the consequences of their actions to get there.
Juniors should absolutely be encouraged to either build a project on their own or collaboratively, or to start by joining the community and listening before starting to write code. This is something that's lost on many and "drive-by PRs" happen a lot on high profile projects from people just wanting to say "I'm an X contributor" on their CV or get a coveted t-shirt and never come back again.
These drive-by PRs come in a variety of forms. Some are just absolute nonsense, some rename things for no good reason, some refactor a part of the code that nobody asked for, and some do add a tiny bit of value, but are extremely bad quality to the point where rewriting it would take less time than going through several (dozen) review rounds explaining to the budding contributor how to write code. (Or how to even use git. Rebasing a PR is a problem for a lot of people, for example.) Maintainers of open source projects typically didn't sign up for being mentors to people who barely know how to code.
More than that, these random PRs increase the noise level in the project by a lot and take away time from the contributors, junior or not, who put in way more effort that a drive-by PR author did. Leaving PRs to rot is bad because they will start having conflicts, so not reviewing them is not an option. Each of these PRs needs to be properly reviewed because they have a maintenance cost later that rests on the shoulders of the maintainers, not the contributor. Each PR carries the risk of someone introducing malicious code. Answering these PRs politely but firmly also takes quite a bit of effort.
Yes, juniors should be encouraged, but realistic expectations should be set. Contributing code to a highly complex project probably isn't going to happen. They should try and contribute to small projects that they themselves know and use. That way they understand what is good an what is bad for that piece of software. Also, they should "read the room" and the contribution guide before jumping in. The best way to start is with small things: reporting issues, fixing docs bugs, and once they are comfortable with the workflow, move up to writing code.
Unfortunately, communication is hard and getting this message across ("be considerate") to the right people is harder. This post has a good analogy for that with the bricks.