I'd assume because pigs have to be fattened before slaughter. At the small scale by literally throwing them scraps (pigs eat nearly anything edible).

Meanwhile in a pig butchering scam you have emotionally desperate people (who often would go for figurative scraps) being fed fake romance before being slaughtered (having a lot of money extracted from them, which usually ends in the end of the fake relationship as results don't materialize for the victim, e.g. the lover not showing up)

Because when you buy a live pig, you don't just slaughter and eat it right away, you have to fatten it up over a period of time and then you slaughter it and have a feast. A typical scam is more like fishing, where you catch a fish and then eat it right away. Pig butchering requires a prolonged relationship with the victim to get a higher payout for more work.

It’s an English translation of a Chinese phrase. The method began in China before it came to the west. “shā zhū pán (杀猪盘) translates literally to "killing pig plate" or "pig slaughter table”.

It's just analogy which is useful to depict process well: pig needs to be fed and taken care of for some time before it's butchered