"Lets be friends first and see where that takes us..." In theroy it sounds good but in reality if you are truly "into" someone, strongly attracted them, or feel a large does of chemistry whenever you are around them...odds are you will not be looking to become "friends". You are going to push for some semblance of a relationship.
The last thing you would want to do is to blow an opportunity by not letting them know you are "romantically interested" in them.
Generally speaking when someone says, lets be friends first it's because they don't instantly see that person as "relationship material". If they thought the person was "hot" they'd want to take them off the "open market" before someone else snatched them up! Last but not least it is difficult to make the move from "friend" to "romantic love interest". It's human nature to put people in (boxes) when we get to know them. It usually takes something major to get us to look at them in a different way.
Naturally there are exceptions to the rules but most people who are "good friends" tend not to want to risk messing up their friendship by becoming romantically involved. It's much easier for your significant other or spouse to become your best friend than it is for your best friend to become your spouse or significant other.