It hurts your reputation a lot for a PI to be a first author.
More importantly, what can you give your students. Biologists are hardest
working and lowest paid jobs. All you can give them is credit for their
hardwork, which is the basis for their career development and scientific
pursuitness. It is very mean for a boss to be a first author, I only see
some colleagues in China doing that, mainly due to the unique system
in China. Sometimes you cannot balance the credit within a multi-level
hirachy system, where the 'big' boss wants to be the corresponding, then it
ends up the Associate professor to be the first author. It is very rare in
US, though I know a guy does this and feel it as a shame.