Ahh, but they are cheap enough to buy new ones when they get brittle.
Over the lifetime of the vacuum bottle, it's cheaper than having to keep replacing the plastic containers. (Although I will admit, I use the plastic at home for soup storage in my fridge)

Also, if you forget to go from rack trunk to refrigerator in the office on friday, then go out to lunch instead, then come in on monday to realize you forgot about the food still on your desk, you don't feel guilty about throwing out the container and all!
Landfills, people. If you're throwing these things out, they're sitting in a landfill until space aliens blow up the earth. (at which time, they will float endlessly through the vacuum of space as interstellar litter.)

If you're going to pitch them out, at least put 'em in the recycle bin.