If you are talking like "my friend's mom" - it's natural for humans to desire first what is within their reach. Young men can get attracted by their friend's mothers and fantasize about them.

Also, such young men who fantasized about friend's moms can later, when more mature, be attracted to MILFs simply because they remind them of those old fantasies.

From a practical point of view, single moms often fuck great. If the kids are old enough those mothers don't really care about catching men for financial support, they just want to fill their lives with something and if it happens to be sex, they are usually quite dedicated to it.

When I was younger I was with two women (not in the same time) who were 10 years older than me and in both cases sex was awesome. They were very enthusiastic, creative and open-minded. Both had kids over 16 y.o. so they did not really look to have a partner who would support them financially. They just enjoyed sex and spending time with me. I was not looking for older women specifically either, just so happened that they were more interesting to spend time with than most of women of my age.