They have always been here, but perhaps your ad-blocker stopped working? Or perhaps you read the forum without logging in which reduces some ads.
It is fun to mention something in a thread and then see all kinds of advertisements for that something. For example, I will mention "Fisher Investments" and now this whole thread will be filled with ads from Fisher Investments.
Why is it done? Well, forums like this are cash cows and probably generate upwards of $100,000 a year in advertising revenue to the owners.