Short answer: Because you want the highest-quality gear, and we want to create the highest-quality gear.
"Wanted to let you know the bags worked great... I did a 58-mile overnighter last weekend, 95% singletrack, a dozen creek crossings, and it rained for the first 4 hours of our ride. Everything stayed rock solid on the bike, and just about completely dry despite the conditions we rode through... Thanks!" -Jason R., Georgia
We are often asked whether our bags are completely waterproof. Almost all the materials and closures we use are waterproof (frame bag zippers included), but our bags are sewn products, because sewing textiles is the strongest way to build a bag. Under heavy rain events (or submersion), you may see some dampness in our bags. Details:
1) "Waterproof" doesn't mean no water in your bags. Camping gear gets wet, and then goes into your bags, no matter what, even if you have "fully waterproof" bags. If you haven't taken precautions to keep critical items dry, those will then become wet. Wet gear examples are numerous and include rain gear after the storm, a weather- or condensation-soaked bivy or tent, sweaty clothing, etc. In short, you still have to individually waterproof (with silnylon dry bags, or even simple plastic bags) sleeping bags, down jackets, and so on, regardless of whether your bags are truly waterproof. This is something we've both learned for ourselves and heard from many other long-term bikepackers.
2) Lower-quality materials and assembly. Making a completely waterproof bag generally means a bag that uses radio-frequency-welded construction. You can't RF-weld the materials we choose to use for our bags; the materials available for RF-welding are generally PVC-tarp-like and not, in our opinion, very durable. We feel these materials don't hold up well under consistent use. Also, a welded seam is nowhere near as durable as a sewn seam. (It is worth noting that while there are a few truly waterproof RF-welded bags on the market, many RF-welded bags also have stitches through the tarp, rendering the "waterproof" claim moot.) Finally, a case study: A leading American manufacturer of motorcycle gear makes both sewn and welded bags. Warranty on the sewn bags is 10 years; warranty on the welded is only 1 year. 'Nuff said.
3) Lower-quality product and outsourced jobs. The technology to RF-weld largely belongs to Chinese manufacturers at this point, and the bags we're seeing come out of China are as expected - poorly made. As a company committed to domestic manufacturing, even if we decided that RF-welded PVC-type materials were the way to go (which, thankfully, we have no reason to believe), we wouldn't go overseas, for both philosophical and practical quality reasons.
4) Finally, you can certainly seam-seal our bags if you so desire! Remove any stiffeners, flip them inside out, and apply tent seam sealer to all stitches. This is similar to US-made Henry Shires TarpTents, or European-made Hilleberg tents - of highest quality, but not seam-sealed due to prohibitive extra cost and tighter environmental regulations in "first-world" countries.
In short, if you're doing a beach-riding, packrafting tour where you’ll be submerging your gear, you may want fully waterproof bags. But for typical bikepacking and touring trips in all weather conditions, we genuinely feel the higher-quality materials and construction techniques far outweigh any benefits of waterproof gear, especially once you consider the need to dry-bag critical items regardless of waterproofness.