Not just new hires. They are firing people on "probationary" status, and people in civil service go through a brief probationary period after being promoted or moved to a new position.[1] This means some people being fired are long-time senior civil servants with expertise and knowledge. The reason they are firing probationary people is because they are easier to let go, by civil service rules.

I suspect the people in charge of the firings are under the same mistaken impression as you are, that all the probationary people are new hires who aren't yet essential. Witness the "oops, we fired the wrong people" rush to rehire.[2][3]

1 https://www.npr.org/2025/02/15/nx-s1-5298182/trumps-probatio...

2 https://www.bbc.com/news/articles/c4g3nrx1dq5o

3 https://www.nbcnews.com/politics/doge/usda-accidentally-fire...