The answer to your question is partially in the *name* of the acronym you are asking about:
MILF: "Mom I'd LIKE TO FUCK"
Now as far as the "obsession" part goes, for me it's because I see being 18 and in great shape (male or female) as a relatively easy process. Now, let a woman have a kid (or more) and get past 18-22... if she still looks like Victoria978 (purely as an example) then you know you're with someone who's extraordinary. Think about the percentage of girls who are attractive at 18. Then think about how far that percentage drops at 30+ -- it's scary. I bet it's at least a 10x decrease. It's probably the same for males as well.
So, you take someone who's kept themselves up beyond the 18-22 age range, and they are more sexually knowledgeable (either via experience and/or research). They know their bodies much better and know what they want as well. MILFs can be a *very* good deal!