From an investor's perspective, why would I invest in the middle when I can put 90% in super conservative securities and the remaining 10% in super speculative

From an investor's perspective, why would I invest in the middle when I can put 90% in super conservative securities and the remaining 10% in super speculative startups? I get all nearly all the upside (unicorn) with very little downside (-10% at worst). (Yes, this is Taleb's barbell strategy.)