If you want to store them encrypted, then chrome has to ask your master password every time you want to actually use the stored passwords (i.e., open a random website). If you don't store them encrypted - then your passwords (as well as everythin else) are available to any random person who has access to your computer for 3 minutes, and not showing them directly is just a minor inconvenience.

Actually, the solution is simple - don't have random persons access your computer for 3 minutes; always locking your screen when leaving is the simple and effective way to counter this risk and many others.