It's mostly inventory and distribution issues. The stuff is hard to carry and maintain stock because of the number of SKU's one has to keep on hand just to cover like 70% of vehicles.
When I was the Buyer for a bike shop chain, I tied as much revenue to Thule as I did with most bike brands, but it hurt in terms of inventory turn times and cash flow. I had most stuff in stock all the time, but that money was not turning margin as well as it would have in accessories or even bikes with their poor margin.
It always made me nervous too. Bikes I can clear out at cost. Clothing and accessories I can swap meet. There is no demand market to liquidate Thule stuff. Like I knew eBay intimately. I can sell ultra high end stuff on there as real auctions with a $0.99 starting price, 10 days, and no reserve. I will always get higher than cost with my techniques (at the time). That would not be the case if I tried the same thing with Thule. A great deal in an enormous market is irrelevant when the product only works for 5% of said market.
It is just a tough product to carry at the retail/distribution level in a way that is profitable.... Just to explain in WAY too much detail. The only way to fix it would be the standardization of vehicles.
I get that. My low level gripe is just that if someone is gonna pay $600 for their bars, it should just come with cylinders in the kit. No relatively sane person is gonna leave that investment unlocked.
IIRC it is done so that you can choose to match the locks upon purchase, buy replacements, stores are motivated to carry lock cylinders, and you can expand based on what you already own. (Don't shoot the messenger :)
It probably also has to do with how contract manufacturing works.
This is the reason. If you have multiple or existing ones you have the ability to match the lock number to use the same key for existing and new locks. If you buy different items that require locks you buy locks with the same number straight away.