They do this because they're not traders but influencers, they only claim to be traders to connect with their audience as that's the perfect way they can use to influence them.
The fact is that some traders do live trading, if their portfolio is good and can be confirmed, then more and more people come to subscribe which makes them truly influencers by their profession. Affiliate programs are one form of passive income that can't be ignored when it comes from large companies.
Most real traders don’t care about passive income from referrals. How many Wall Street traders do you see who are publishing their affiliate links to make an extra 1%.
The ones who spam their affiliate link usually do it because they want free money for doing nothing. They pretend to trade or trade with small units and trick people into think they are some great traders.
Happens all the time in almost all markets.