If Goldman had a way to guarantee returns above the market why would they sell that to anyone? Or put it another way, who is the sucker that is taking the other side of those Goldman trades to get returns guaranteed to be below market? Total market returns are zero sum.
In general (not answering this specific case), a fund wants more money under management, because they get percentages of more money that way (they have more leverage). You might be assuming Goldman has infinite money - they don't. Here's how to think of this:
Say I'm an investing wizard - I can generate 20% returns on any money invested.
Scenario 1: I have 100k capital, which means after one year, I'll have 120k. I made 20%, but in practice that's 20k.
Scenario 2: Now say I sell to you my wizarding ability by giving you 10% and keeping 10%. I'm giving you half the upside. However, you are a rich bank that invests 100m. I turn it after one year into 120m, give you half of that and keep half, so you've gotten 10m and I've gotten 10m.
Now percentage-wise, obviously I'm ahead in scenario 1, but in real terms, obviously I prefer scenario 2 - I've gotten 10m dollars instead of 20k dollars.
Btw in the real world, if I have to invest more money, my prowess goes down. So with 100k I can do 20%, with 1m I can do say 19%, etc. So what you'd expect to have happen is that more and more people will invest, because it's worth it to them, until eventually my abilities go down to just generic S&P level. Up to then, it's worth it for every extra investor to invest, because even if I'm only beating the market by 1%, that's a lot, but eventually I'm managing enough money such that I'm just rivaling the S&P. So from the outside, it looks like I'm not really doing anything special, but it was totally worth it for every investor until now.
