I might be wrong but it might have something associated with the numerical goals we acheive as children, young adults and grown up. For example, becoming 10 years of age is a sign of maturity, mainly due to the double digits it contains. This leads us (as children) to believe that we are that much closer to becoming adults. When people turn 40 or 50 they put a lot more pressure on them selves than they would have if that number was decreased a year or two. Maybe it's the obvious facts such as 50 is closer to 60 than 49 is?
Also, when we learn how to count, we don't necessarily stop at 3, or 7, 10 is usually the number that kids are first introduced to, and once they learn it get appraise from external figures.
Basically, our preffered simplicity for numbers may or may not reflect our own personal achievements during our childhood and other significant life experiences. I knwo for a fact that I wouldn't be as proud of running a 39.7K as I would have been for running a 40K, even though the difference is a tiny 0.3.
When we announce our achievements to others we say "I'm in the top 10 for my age in cross-country!", assuming we come 7th, top ten sounds like a greater acheivement, this is just me, others may have different opinions.
Growing up and being congratulated for our achievements by older family members and other adult figures for unspecified winnings might have an impact on our greater admiration of "rounded numbers". I'm not quite sure whether or not I explained my self very well but I hope you understand what I'm saying.