Gambling tax is still because it's pretty much unenforceable. It's like taxing unrealized gains, it makes no sense in practice.
A very small amount of poker players, only those who do it as a full time job report earnings. Even then, there is no way to verify losses. What is the definition of a loss? Do u have to track every hand I lose, down to folded blinds?
Do I track losing sessions? The definition of a session is completely arbitrary. I could be down a few hundred, go to dinner for half an hour, the come back and end up in the green for the day. Do I have to separately track them separately and get taxes extra before dinner?
It might be easier to keep track of for tournament players, but they often fire multiple bullets in tournaments. It i buy in for 1000 5 times, and am end up winning 50,000, do I have 4k in loses or is it treated as a win overall.
All of those questions don't matter because noone is going to accurately report their earnings when there's a tax incentive not to.