It’s really not that terrible for occasional purchases. Sometimes people just want to buy some meat of reasonably reliable quality (and that’s been refrigerated) at whatever time to cook a quick meal, maybe while picking up a few other items.
Sure, it might not be the cheapest place in Taiwan, but not everyone is as conscious of prices as you are. For me, paying NT$90 or whatever for two vacuum-sealed chicken breasts (300 g, or NT$300/kg) is totally fine.
It would be cheaper if I went to Costco (around 35% cheaper – they were NT$195/kg as of yesterday) or a wet market (no idea how much they cost there), but then I’d have to deal with going to Costco or a grimy wet market, and I can’t necessarily be bothered spending my time doing that to save an amount of money I’m not particularly concerned about. For those of us who work, our time also has value.
Some of the Carrefour stores near me also seem to have the habit of overordering these, such that when I go in at 3 a.m. or whatever on the way home they’re often heavily discounted, in which case I might buy three or four packs and cook pulled chicken or something the next day for freezing.