It's like this because of 2 things: people's general high willingness to pay for CX and HK Express

1. The fact is, people will still buy the flights at this price; the rest of which is supported by CX's long haul network with connections. Together, these flights will fill up.
2. For those travelling from HK like yourself, they have the option of choosing HK Express if they are price sensitive. If CX is too cheap, no one would take HK Express.

And as it turns out, Cathay's flights are generally fairly full.