why there isn't the superman symbol in steel's chest?

because it's a movie not a comic book and they never made a reference to why the symbol would be there like in the comics so ther'd be no point in having in the film. besideds he had the tatoo on his arm. that i thought was good enough.

RIP:
Heath Ledger - 1979-2008
'You'll be missed'

You're obviously not familiar with the solo Steel series. He removed it. Eventually it got put back after the costume's third or fourth redesign. Besides, in this movie there's no real mention of Superman existing, so to have the symbol wouldn't really make sense.