Well maybe I’m wrong, but I suspect he is aiming at a higher office.
But being mayor of a big city can wear a politician out.
No mayor of New York has moved on to higher office since Reconstruction (though a former acting mayor did make it into Congress in the 1920s).
I suspect mayors of Chicago are more likely to be assassinated than to become senators or governors.
And it looks like Illinois governors are more like to go on to prison than to any other elected office.