Why are hedge funds usually started by 1 person?

A PE fund is typically going to hold investments for a long time and can't liquidate them easily. Ownership is also operationally intense and sourcing is often (though decreasingly in late stage PE) relationship based. This means that multiple partners provide bandwidth and redundancy. It also reduces the dynamics of partners tearing each other apart when they disagree on something in the portfolio. You can't quickly sell it, so you have to make the best of it.

Hedge funds with short books or other forms of hedging require very rapid reaction (a short moving against you hard can get very serious very fast for instance). Most HFs that are started by multiple PMs wind up failing becaues the two PMs tear each other apart. The stress of constant market moves (especially on the short side, where downside is unlimited), the need for quick reactions and the fact that you can always sell a position (and thus, can always relitigate a decision) destroys most dual PM marriages.

Some hedge funds have multiple founders but usually there is one investment trigger puller. Other founders may be senior analysts, business development or ops. Some can have substantial equity but are not the investing decision maker.

LP preferences and experiences reinforce the above. No LP wants to liquidate a PE fund if a single key person leaves, but wind down is not a problem for most liquid strategies. Meanwhile, most LPs have seen dual PM hedge funds fail and aren't interested.

Some long only firms have operated successfully as multi PM. They tend to have a longer term approach and lack the pressure of needing to be as responsive to the market (through the short book or other hedges - trying to beat the market over time is different than promising to preserve capital through all situations, which is a more ambitious goal that some hedge funds are aiming for) and so can have a more consensus approach. Of course many are not but you could think of long only as being in the middle in terms of most of the forces mentioned above.