Maxing my Roth IRA and Traditional 401k feels just as irresponsible to me as gambling on slot machines.

Here's the thing: you don't know if that "safe" index fund you put money into is going to continue going up or crash straight to zero.

I've lost a lot of money gambling and buying individual stocks, and I see zero difference. At the end of the day no one has a crystal ball, and past performance does not indicate guaranteed future growth.