Most Helpful Guy
One problem is that you become invested in the person as a friend rather than as a potential mate. Love relationships are inherently risky, and after a few years of being 'just friends,' neither person wants to risk their invested friendship by risking taking it further. So even if feelings develop, they can go for years without taking the next step. When it's strictly on a relationship basis, it makes it much more simple. If you like each other, great. If not, part ways and move on. No messy friendship residue to have to put up with.
From personal experience, every girl who wants to be friends first with a guy simply isn't ready for a relationship. She's either too immature or too insecure to say, 'Hey, I like you, and I'm willing to say it straight.' I don't waste my time with girls who are indecisive.
Most Helpful Girl
I wouldn't call it being "friends" .. because when you say it like this guys can just take it like "just friends"
But I totally understand you. Some guys just wanna go straight to it before actually getting to know each other. I don't understand why they do this either (some and not all guys) but my guess is that they either want the relationship for sex. If they don't care about your personality... or they just want it for the sake of being in a relationship- and not because they like you in particular. Meaning they will go out with any random good looking (to their taste) girl who would agree to be in a relationship.
I personally wouldn't agree to a relationship before getting to know each other and even if a guy shows me impatience I'm only gonna take it as a red flag.