If their ads said that negative ROI are likely, they probably wouldn't get much new business.
People still bought stocks and mutual funds in 2009 even though investors had a negative ROI in 2008 and some indexes (like Nasdaq) are negative over a 10 year period.
Some people had gains and some had losses--there's probably a bell-shaped histogram of mutual fund returns out there.
I don't see NYSE/Nasdaq running ads claiming 6-14% returns on the stocks on their exchanges.