Easy.

1. Tell them it will keep you out of the bars, off video games and save you from the endless mindless hours spent on your smart phone.
2. You can charge your friends to play and easily make back the money spent on the machine.
3. Pinball machines are stone reliable, so you'll never have to pay to fix anything.
4. Pinball machines only appreciate in value, so once you tire of it you can sell it for big bucks--way better than investing in the stock market or a college fund.
5. Since pinball is a dead-bang money maker, you'll want to dedicate a whole wing of the house to pinball.

I could go on, but that should do it.