Simply because his unfailing logic would force him to look at the options and make the right choice.
Choice #1 - pay someone else to 'manage' your money and skim 1-2% off the top every year regardless of how they compare to the market averages
Choice #2 - 'manage' your account by trading constantly, making moves based on the daily news, buying into newsletters that claim they have the 'picks' you need to beat the market and/or buying some crazy software for thousands of dollars that can help you beat the market
Choice #3 - dollar cost average buy into a very low fee diversified ETF or mutual fund as often as you can
Spock would have no problem figuring out that the logical choice is #3. He knows that simple strategy will have his portfolio in the top 10-15% of all investors and will have achieved it without spending valuable time, money or stress.