why do you think forex is harder than the other asset classes?
It isn't.
The huge difference responsible for the very different collective outcomes you've observed (and about which you're definitely right)
isn't directly because of the differences between the asset classes: it's mostly because the trading participants of those different markets are
different groups of people with different approaches and attitudes.
The entire world of retail forex "brokers" (they're not actually brokers at all - just counterparty market-makers
pretending to be brokers) is designed and set up specifically to attract an enormous turnover of customers
most of whom are destined gradually, inevitably, inexorably to lose: typically they're undercapitalised, over-optimistic, prone to gambling, attracted by "free bonuses" and competitions, they don't really understand what a broker is, they're typically undereducated (at least in the ways directly relevant to what they're trying to do and how to do it), they're overleveraged, and they tend to have unrealistic expectations which lead to more or less deluded perceptions of what they hope to achieve and how quickly. And the "brokers" concerned are highly skilled at targeting and attracting them, because that's how they make most of their own livings.
Most aspiring forex traders significantly overestimate the extent to which they can make profits quickly (while also actually significantly underestimating how well they could potentially do much more slowly, with the requisite patience and discipline, by developing real skills built on a foundation of numeracy and understanding/experience of the generally counterintuitive worlds of probability and statistics).
"Investors" are - by comparison - far more concerned with capital preservation, and are less gambling-oriented, and unsurprisingly they tend overall to do better.
The asset classes with which each "group" is engaged are necessarily different because of the ways their markets are structured, thus giving the misleading
appearance that one asset class is "harder" than the others. In short, the facts you've oberserved are indisputable, but the reason you (and many others!) have attributed to this phenomenon really isn't quite right. Tom's post above, however, is exactly right.