It depends

If a guy is still single at late 30s with good income they are usually one of the following:

1, not goodlooking or not in good body shape

or 

2, Very introvert

or

3, strong personality 

or

4, They set in their own ways. At this age they do not want to compromise for anybody or anything. They are the kind that happy to be alone and go on some dates or maintain a relationship but not marriage.

I know all of four types of single middle aged men. #2 is the best.