This has been an enduring mystery to me: why sell your winners instead of letting them run?
@DaveT has the
best explanation I've seen to date: ARK Invest has $11.1B in AUM (assets under management) as of Nov 2019. They charge 0.75% management fee for their funds.
The way ARC Invest makes money (as opposed to their clients) is by beating the performance of the S&P 500, thus attracting customers away from passive Index funds like Vanguard. For example if ARC could grow to $13.34B AUM they'd make $100M per year in fees.
That's their underlying financial motivation. They don't make much extra income if the SP goes up, they do that mostly by attracting new investors. For the rest of us Tesla-oriented investors, we're better off buying shares directly.
Cheers!