i can see you coming from a fairly religious mindset and i agree that from a religious POV even speaking to a na mehram unnecessarily is not appreciated. however, so many people who view girls that have male friends or who have lunches with male colleagues are, not always religious minded neither in their views not in practice. it has more to do with the narrow mindedness than religion with them.
the valid reason for this is that families that are practicing muslims and are following shariah rules would go for the rishta of a girl who belonged to the similar religious family as theirs.
also the very families and guys who would refuse a girl because of this are the ones who find no problem in free mixing of cousins of opposite genders. if such people are to raise the point of na mehram for work place colleagues then why does that goes out of the window when it comes to cousins? Again, I am not talking about the shariah observing families here, but the average families with moderate religious views.
And with all due respect, even if you are somuch religious or a practicing muslimah, you still can not view girls havingmale friends whether at school or work, as immoral. As in Islam everything isbased on Neeyat