You don’t have to invest individually to take advantage of dividend paying stocks (i.e. investing in an ETF like DVY, which currently has a 3.16% dividend yield – almost 4%). And while your math is indeed correct, there is more to dividend paying stocks that just the math. The reason the companies pay dividends is typically because of their underlying strength, steady growth, etc. These companies can be good investments for the long run. As such, it might not make sense to sell. 

What does passive income mean?

However, this comes back to the old discussion of pain versus pleasure. We will always do more to avoid pain than we will to gain pleasure. When our backs are against the wall, we act. When they're not, we relax. The truth is that the pain-versus-pleasure paradigm only operates in the short term. We'll only avoid pain in the here and now. Often not in the long term.

How can I make extra income?

The robo-advisor company charges a 0.50% fee and on top of that there the fee charged for the ETFs (which is anywhere from 0.25 to 0.5% as well).  Robo-advisor companies help you rebalance your money automatically so your original asset allocation is preserved.  Basically, you can be completely hands off and all you need to do is funnel your money in there and they will invest it for you.
The United States Internal Revenue Service categorizes income as active income, passive income, or portfolio income.[1] It defines passive income as only coming from two sources, or "passive activities": rental activity or "trade or business activities in which you do not materially participate."[2][3] Other financial and government institutions also recognize it as an income obtained as a result of capital growth or in relation to negative gearing. Passive income is usually taxable. 

What means passive income?