I asked myself the same question and it certainly is possible because players have done it. I suspect it’s not an easy feat for many reasons.

Poker is like a pyramid scheme in that the sharks feed off the fish and because there are more fish than sharks, it’s easier to move up at lower limits. But, as you move up, the number fish goes down as does your winning rate before you likely reach a level that you can’t get by and now, you’re the fish. Also, as you move up, the games are tougher to find, which can cut into your efficiency and profitability.

I came up with this chart detailing the progression to become a poker millionaire:

Bankroll is the money you should have to play at that level. Buy-in is your bankroll divided by 30. SB is Small Blind. BB is Big Blind. Delta is the difference in bankrolls between the current level and the next level – the amount of money you need to profit to move to the next level. Delta Buy is the Delta divided by the Buy-in – the number of buy-ins you’d need to profit to move to the next level. You should never risk more than 5% of your BR (Bankroll) per day.

The first thing I notice is that the smallest NL buy-in at a casino I've seen is the $200 NL $1/$2, which requires a $6,000 bankroll. How many people at the casino playing No Limit actually have that capital? Probably not many.

If you can make it to level 15, you’re a poker millionaire. But, again, that’s no easy feat as for every dollar made for one person is a dollar lost by another person, so for a person to be making millions, someone(s) has to be losing millions. For one person to get those millions is economically possible, not probable.