I think it's that so much of the appeal of Romance as a Plot for me personally is "these two people's personalities bring out interesting things in each other," but if it's just instant attraction right away there's no time to get around to the personality!*

*I make something of an exception for 'we knew each other long ago and have just re-met' or 'we knew each other long ago and forgot but remember a little' because there at least there is a little bit of an existing personality pattern to draw on