The bias is called common sense.
Yes,
common sense says that these people must have a life outside of their function as a motivator for the protagonist.
However, we never get to see it. We never get to see how
they deal with a crisis. We never get a glimpse at what they do outside of their interactions with the protagonist.
They are not presented as complete characters. They are a cipher, and every hint of a deeper life these characters have is mostly in the mind of the viewer.
That is the core of the argument here. These characters, although they are presumably very important for the protagonist, never get fleshed out as well as he is. They're never treated as equals by the narrative.
There are a bunch of issues that arise from this. For example, while the MPDGs are generally totally interested in the protagonist, he never has to spend anywhere near the same amount dealing with her feelings, or helping her with her problems. It's not a balanced relationship, and that's a dangerous thing to portrait as something positive.
For a subversion of this trope, I recommend the film
High Fidelity. In it, the protagonist is dumped by his long-term girlfriend, and tries to get out of the ensuing depression by dating a bunch of what he hopes will be MPDGs. However, as it turns out, all of them are much too complex, much too adult to actually fit the role (Not that they want to, them being actual characters and all that), and so he is forced to actually deal with the real issues he and his girlfriend were having. Now, while Laura isn't given anywhere near the same amount of screentime as Rob, it is still clear that she is a full person with needs and wants and a life of her own; something none of the characters commonly referred to as MPDGs are.