Its actually NOT,
What you are confusing is the sale booking price and full fare.
There are many Economy fares starting with Y, then going down cheaper, B,H,K ect... when you see $2500 to HKT on KA, that is a sale Economy fare.
Full, unrestricted Y class is around $5000HKD + taxes and fees
a economy Z ticket is around $6-700 (not sure exactly as I get Business tiks)
in essence your paying about 10% of the FULL FARE.
But you are correct. MANY times you can buy confirmed SALE fare tickets for little more.
EX. Last year I had Z fare from X to Y to Z. Required 2 coupons and was about $1200 HKD. I saw online the night before the carrier had a sale, and was able to purchase tickets for 900HKD. I just kept my Z tickets and refunded and bought confirmed seats. I was Star Alliance "gold" frequent flyer, got priority tags, used lounge, it was perfect.
The one "benefit" of Z fare is flexibilty. You can TRY to come and go as you like. If you purchase a super low Y fare, often there are no changes allowed.