We need to first define as to what a 'nice girl' and a 'nice guy' are. What traits that many if not all can agree on that makes one 'nice' to begin with. How does one determine whether if they truly are nice or not, and if nice is the proper term to describe someone who does the things that they do and why they do them.
That being said, at least from my understanding, if someone claims that they're 'nice' then it has to be proven to the person(s) they want to convince and the intents behind of the term of them to be considered a nice guy/girl. More importantly, if you want to be considered nice is it because you voluntarily and convincingly do what's good or right to the people around you or are you doing so because you expect something in return? If we're truly honest with ourselves, I think setting apart and looking into ourselves and what you truly want might not make us 'nice' to begin with, or at least, saying one thing but subconsciously become something else.
Reality is that often times one has to be boldly rude, dishonest and stomp down on others if they want to get ahead in life. I think we can agree that many of how or why people become of higher status may do so not because they follow the Commandments thoroughly, let alone believe. Therefore, the 'nice' people even when it comes to dating scenes find that they're not excelling at it because the vast majority find other things that are suitable within a partner/mate and/or they find suspicion within the individuals who try doing good and think that in the long run they want to obtain things that aren't as Godly. Sad to say, I can't say that isn't true either. That is also why I would question of someone who insists that they're nice/genuine but are they expecting something in return for it? Would their actions prove of what they say and do they carry it out to the full extent, or just for the sake of something that they want?
So perhaps the 'nice guy/girl finishes last' CAN be true, but we need to find what classifies as being nice and why we are considered nice as to the comparisons made of those around us and see if there's any other stumbling block that we may or may not want to admit and find it easier to blame the next guy/girl on. Feel free to disagree with me.