I think, like a lot of others have already posted, that there's really no one good reason for it, but rather a cumulative effect.
Times change, and with that methods, and what's acceptable in terms of design and use. If you look at how things have trended, we do seem to be heading in the direction of convenience and speed. Speed in working with a knife, often leads to breakage or accidents. Convenience of carrying one tool to do multiple jobs is another facet of it. Just look at the market, not only for knives, but everything; If it only does one thing well, most people shun it, in favor of something that has 50 applications that it does moderately well.(iPhone anyone?)
It also doesn't seem "fashionable" any longer to carry the right tool for the job, or to carry more than you "have to". Multi-tools, ultra-light trends, some guy "surviving" with only a knife, water bottle, and flint? Doable, sure, logical...well that's debatable based on what you want to achieve. Whether you want to immerse yourself in the crafting, and working with the tools, learning skills, or just want to go as fast and light as possible, or just feel like carrying what appeals to you.
In a practical sense though, you have true users, who might choose "overbuilt" for valid reasons. Thicker stock, means you can get more sharpenings out of it. It also reduces the chance that it'll fail, thus it's a one time purchase, or at least fewer purchases. In a world of disposable, true users from a by gone day, still like that you can get one good tool that'll last.
Ofcourse, we could go on and on about this. Eventually, like most discussions here, it will all come back to personal choice. Whether those choices are motivated by socio-economic trends, fashion, aesthetics, practicality, etc. ad infinitum.
Then again, that's just my take on it. I'm just a poor, uneducated, country boy.
Gautier