How does "call us back when you get to series A" entail "none of the risk, all of the upside"? The risk is that the fund has to pay a higher valuation for the same company, they might not callback, etc. and the upside is that you're...not taking on the risk right now?
Secondly, for tech hiring, a few bad hires can ruin a company. But, angel investors and small funds generally invest in a wide range of startups knowing full-well that most of them won't succeed. But, one success is all that's needed to 10x the value of the fund. Given the payout probability and payout value of startups, it's a pretty different game from tech hiring.