Because they thought they will get some profit by using other trader's service.
Indeed there are some services like that, we can invest on the project which trades on the market.
But there are many scam trading services as well, they just want to get our money and run.
Yes, the world of trading is also very risky and many also make scams to get money from other people, so to avoid this kind of thing, it's best not to trust third parties that we don't know at all.