We do have an issue open for this feature request:
I definitely understand the desire for this kind of feature. In fact, a lot of people are using external password managers for that very purpose.
There are two main challenges with implementing such a feature in the browser:
The user experience is tricky to get right. The Firefox equivalent for example has gone through several iterations and has never been something that the design team was happy with as far as I know. There are also some considerations around phishing.
In order for the master password to be useful/effective, the password database needs to be encrypted at all times while the browser is running (except when auto-filling the password). I have not looked at the Chromium code, but I suspect that it assumes that it can read from the password database at all times and changing that assumption without breaking anything might be tricky.
So it’s not something we’re opposed to doing, but it’s also not a quick one.