Because first class baggage is labelled and treated differently so it never gets bumped. The lower/higher numbers are also uses to force the penalty payments, something first class travellers don't want to deal with at check-in.
This seems unrelated to your original point about safety of baggage handling workers. The purpose of a max single bag weight is to facilitate handling. The purpose of a per passenger baggage allowance surely is to manage total plane load, hence high excess charges.
Even first class passengers can't turn up with unlimited luggage but, in my experience relating to families flying between London and the middle East, have household staff who make arrangements and coordinate with carriers.
