Taking a stab at it –

Since the abdominal muscle mainly stabilize the body – I’d assume that the endinous inscriptions and linea alba play a role in this function. They join the muscle to the bone right – namely the ribcage there by holding the muscle in place?

Since evolution would favor the minimal recourses required to do this job satisfactorily, one can assume that how we find them today is that result.

(probably one of numerous possible results determined by genetic drift and or the presence/availably of particular random mutations).

Hope this idea helps.