The average assumption is that junior developers are hired because they're cheaper, while there's some truth to this we also need to acknowledge the fact that there is a scarcity of senior developers coupled with the fact that the industry won't evolve if junior developers don't grow.
One of the biggest hopes that employers have in junior software engineers is that they're hungry, that they aren't comfortable with their current skillsets and would love to grow quickly. Unlike other sectors you can grow on your own even outside your workspace, you can execute pet projects that will help you learn a lot.
The faith of employers isn't unfounded, you'll hardly find a junior software engineer who isn't pushing for more, when they're not working you will find them studying, engaging in discussions with people who know more, doing everything possible to grow quickly. This is why the pay cheque of a software engineer can grow exponentially over a short period of time.