No, the actual answer is because they don't think the people who are buying these things will perform a careful analysis of features and select a product accordingly. They figure they'll pick a bag based on brand, color, marketing, flavor-of-the-month gadget, etc. They figure that because they're usually right.
It's not about working outdoors, because, frankly, most work with tools does occur indoors, but a lot of people who use tools (professionally, especially) are, whaddaya call it, itinerant. You're always going to and from the jobsite, and you've got to get your tools from the truck to the house or building and back. The nicer toolbags are padded to protect your tools, but they don't do shit about rain.
And your typical carpenter goes out to the jobsite with $500 in cordless tools, easy. Rain's not good for them. I have to take my tools to work at least once or twice a month to fix something or other, and even I often have close to $200 in tools in my bag, and it seems like half the time I have to walk three blocks in the rain. I usually use military surplus bags instead of tool bags because they're more weather-resistant, but I often put the tools inside a bundled-over Hefty bag in there as insurance.
The same goes for a bag that has your computer or important papers. I dunno. Some have flapovers, but apparently it's just random whether they put a good one on there or not.
