Diversification is for people who have no idea what they're investing in. Portfolio theory is spray/pray with no information which is what VCs do with unestablished startups, hoping for the wins to beat the losses. If you want to be that passive then just buy an ETF or all the large-cap blue-chip dividend stocks instead to keep it simple.

Investment funds with a real thesis and research don't do this. Concentrated positions and proper risk management is active investing and generates much greater profits. If you know a sector and company is doing well, diversifying will only reduce your returns.