The reason usually given for having remote junior developers is cost savings. People in the decision-making chain needs to understand the effort involved to make the cost savings possible and not assume that offshoring automatically equals big cost savings.
I (based in North America) got a successful team of junior developers in Bangaluru to be self-sustaining as a remote entity, but it takes work. I went to India and trained a team of eight developers for three weeks. During this time I got a sense for how each worked on their own as part of the larger team (helping their co-workers, taking leadership roles, etc.). Then I whittled down the team to the four who made the strongest team. When I returned to North America I committed to pairing with remote team members via web conference during their business hours (middle of my night). The other US-based senior developer and I did code reviews of all changes made for several months, always requiring robust unit tests for any committed code.
So it was a lot of work but we were able to retain the team for a few years and it probably saved a little bit of money.
As stated in other comments/answers, mentoring less experienced resources is important, and doing so remotely is more difficult than doing so in person. But it is possible.