If you put in the maximum amount, it would take over 50 years to put in $1 million. The only way to have a million in a 401 is to put it in equities and hope there isn’t a long slump in the market along the way, or in real estate.
Or start maxing out on the contribution at 20, and still be pumping in at 75, with no periods of unemployment and no financial emergencies.
You have to consider the employer's match, and the rule of 72. It really isn't that hard to get a $1 million over the 45+/- year work span.