Number of rookie Secret Service agents rose as veteran members dwindled in years leading up to Trump shooting

[Igel]

3. Who else do you hire?

You can't hire the people already working for you--"old hires".

The budget issue indicates the reason isn't an expansion of the corps (which would also lead to a new hire bulge). On the other hand, between budget, Trump, COVID, and the lack of a need to go into law enforcement when the mood in 2021 was to look down on the oppressors, there you go. Police forces had the same problem. Even the military had problems (although arguably a lot of the military's problem was fitness, drugs, and drop-outs.)

There's no indication that things changed in 2021. Because if the Biden admin wanted to replace retirees, it would have to, um, replace them with new hires.