more people may be more active in doing both. but have you ever seen anyone think they are only lucky in one area? Everything can be learned, but sometimes people feel that they are more fortunate in a particular field when they have tried two or more times in doing something.
Likewise with investment or trading. people may be able to do both, but sometimes they always fail in investing and on the contrary always profit in trading. Or maybe on the contrary there are those who always fail in trading but profit in investing. So maybe that's the reason why there are people who don't do both.
This is the perfect thing which I also examine that everyone cannot be profitable in every field and the major concern is about the strategies, learning, knowledge and interest in selected field. People are not interested in investment in most of the cases because they are in search of quick profit which is not easy with investment.
Some people on the other hand are not interested in trading due to the quick decision, timing consuming and presence of high risk. People often think about their profit and if they find anything profitable then they will try to learn about that because no one will create any idea about unprofitable business learning.