I believe it has a lot to do with Se. From what I have noticed (I notice great many things) Se types are the most keen on sports and who have the best physical structures. This is not limited to SPs. Even NJs have fairly firm structures and are comparatively more interested in sports than SJs and NPs.
And I have mentioned this before that Se types seem firmer and stronger than Si types.
My ISTP husband doesn't play sports and never has. So not all ISTPs are drawn to sports. However, every once in a while he will want to do something physical like mow the lawn or take a walk. He's told me that it helps center him when he does activities like these.