Given the same investments in each, the unicorn provides a bigger return. Because of economies of scale, it's more likely to gain traction and continue its success, especially in a place like tech where marginal cost is almost non-existent. Not to mention the overhead on managing a single investment vs. many. Not to mention the administrative costs of 5 companies vs one. Not to mention the benefit of choosing a company based on your interests and abilities, since most VCs have a more hands-on role than traditional investors.
It just aligns with the culture; the whole idea in most tech startups is to leverage economies of scale as much as possible. Often their profit per customer is tiny. They're relying on millions of people using the product.