There are multiple, rather convincing, good reasons for this situation:

1. Inferior quality timber IS inferior. It’s cheap, sure, but that’s because it’s extremely low grade timber. These timbers are terrible to work with, split regularly, and warp even more regularly. They’re so genuinely bad you wouldn’t use them for a kid’s icy pole, let alone a house.
2. There’s no contest between good and bad timbers. When it comes to good and bad timbers, the contrast is 100%. “Good” is like a Ferrari and “bad” is like a skateboard. There’s simply no comparison. Top quality structural timbers are typically super-solid and maintenance-free. They’re easy to work with, and excellent as supports. Equally importantly, these first-rate timbers deliver much better value for money.
3. Product life of good timbers is far longer. Another very unambiguous comparison is the product life of top quality timber vs inferior. The top quality timbers will last for the life of the house. The low quality timbers may not survive more than a few years before needing maintenance, and ultimately, replacement, at added cost.
4. Low grade timbers can actually devalue your house when it’s on the market. Any buyer’s building inspector who sees iffy-looking, low quality structural timbers will form an instant and accurate opinion of these timbers as possible liabilities. They will then, just as instantly, reduce the value of the house by the cost of replacing those unacceptable timbers. They may even advise against purchase if the timbers are looking a bit too dilapidated. You just don’t have these problems with good timbers.