Now here are my two cents.
1. Throwing more developers on a problem doesn't always help. For my studys im currently creating HTML and PHP webpages. If somebody else would constantly work on the same files, that im currently working on, I would be really annoyed. Having to resolve merge conflicts would cause more harm then it would help.
2. New people need a lot of help at the beginning. For a certain amount of time new people would slow down the development progress instead of speeding it up.
3. The person who created a mechanic is often best suited to fix it.
4. Its bad practice to hire developer only for a short timeframe where they are really needed and then dismiss them afterwards. Yes sometimes there is no other way. But nobody likes to start working at a new company, learn everything for a month or more, work for a few months, only then to get dismissed.
5. You don't know if they aren't already hiring new people.