Not just in trading but even in gambling too, you see people giving so much value and importance to bought strategies order than those ones they had to gradually build by themselves. And I see why. It's because the source from which they bought those strategies designate themselves to be professionals and expert in the niche and they get overrated and whatever they give out sells even though it still fails.
People don't believe in themselves enough to trust in their own strategies and predictions. I've never heard of buying a trading strategy, but I see it a lot in gambling. I agree with what you said; people believe "professionals" more than themselves, but they should be able to know that things don't work like that, especially in gambling.
A strategy needs a lot of factors to work, so just taking the strategy and those factors are not constant, then it's useless. If a strategy is that good, why would a person need to sell it to get money? He can simply use them and make thousands of dollars daily.