It is, because there's no choice not to gamble. You have to make choices, and none of those choices are risk-free. The options are on a scale between low-risk negative-return and high-risk high-return. But the risk is never zero - not even in FDIC insured accounts - and there's an element of randomness involved in the outcomes.
That's the point. There's no option to say "I have this store of value, and if I don't do anything with it it will retain its value forever."
It won't. So you're forced into risk assessment and randomness, not just with money but with assets in general.
This is the foundation of the economy. All non-trivial transactions are based on risk/reward estimates, and some parts of the economy can force risk and hazard on others.
It's not just a casino you can never leave, it's a casino where the management use various tricks to siphon money from your assets into their pockets without giving you any agency over what happens.
If you don't believe in that, precious metals are another way to go. I don't personally subscribe to that theory but that's the idea.
Allow me to refine my point: It's gambling when you seek to increase your risk profile in the hope of gaining higher returns, without actually doing anything else other than "invest" the money.