I should clarify: it's not that it can't or doesn't do OTF encryption, it's that it's very limited and incongruous with with the usage model.
When your computer is on, it's generally tied to a desk in a reasonably secured home or office building. If you have a laptop and you're carrying it in a bag, it's generally in suspend mode (TrueCrypt can be configured to automatically dismount volumes on logout, timeout, power saving, etc.), hibernation or powered off, and not otherwise polling for notifications like a phone is in its equivalent lower-power mode. Note that the hardware state of hibernation is equivalent to powered off, so any open encryption volumes are locked if you're using FDE or the OS volume is encrypted.
If someone swipes your phone out of your hand or pocket, it's pretty unlikely that it's going to powered off in an encrypted state at the time and your data is in the clear, protected only by a relatively weak PIN. If somebody swipes your laptop bag out of your hand / your car / a bus / a train / etc., chances are much more likely that it will be in an encrypted state and your data is pretty safe.