Why? Conservatives use the word 'life' in exactly the same way. It's not reasoned debate (which has been had ad nauseam) but argumentum ad baculum. Slogans exist for people to assert what they stand for in a recognizable way, without getting drawn into dead-end arguments by bad faith debaters.

There isn't a 'correct' answer you can just reason your way to when conflicts of values are involved, because people with different value systems have different initial premises.