Because then I clean my hands afterwards, and the rest of me.
It's not that hard? Think about it - would you raher step out of the shower with:
A) Potentially piss on the tiles/floor/you/bath/stuff in bath/whatever OR
- B) None of those things potentially splashed with piss, and with you and your shower guaranteed to be clean
Piss in the toilet. Shower in the shower. Don't mix the two in any way.