Where have you ever been re-screened after boarding the first flight.
Just a few examples off the top of my head:
- FRA always has re-screening when you change from non-Schengen to Schengen
- MUC usually has re-screening right before the gate for US-bound flights
- IIRC, SIN has re-screening at every gate
- When you change between carriers that operate out of different terminals, you usually have re-screening because most airports do not have a connected security area.
- Or, of course, if you have to change airports within a city, like the infamous LHR-LGW run