stilton
It's just a different system.
I don't think BA has ever had a tradition of folks joining on shorthaul and inevitably progressing to Longhaul.
Twenty plus years ago
it was perfectly possible to join BA directly from flying single/twin pointy jets in the military ( ie. No big multi engined time) and go straight into the RHS of the 747. BA couldn't get enough internal applicants - in those days the senior shorthaul pilots had a predictable lifestyle, lived near LHR and got home most nights......why stuff that cozy situation up?
I'm sure the situation is slightly different today, especially financially, but even so these days the new entrants have joined BA with previous airline time and are subject to all the rigorous checks and balances, so like you I'd question "getting away with it".
BTW I reckon the youngest Captain I had the pleasure of flying on the 747-400 with was 32 or 33.