As the tax rate increases, the value you get for increasing your income goes down proportionally.
I think if you take it to the limit, you can see the effect more clearly. As the tax rate approaches 100%, say 99%, then even if I make $100,000 before taxes, I'd only keep $1,000. If I push myself hard to earn $200,000, I'd get to keep $2,000. Yes it is still double, and I am doing better, but not much of interest changes in my life and I personally wouldn't say I'm doing well. I can barely afford to take that second vacation, or see much value for my efforts. My efforts are simply less rewarded, giving me less tangible reason to make the extra effort. I think this would kill off the incentive to create wealth in the economy.
Wealth is needed to take care of basic needs, so it would be self defeating if you were trying to take care of everyones needs and make sure they had a little spending money to participate in the economy.
Somewhat as an aside, I didn't consider what benefits I would receive for paying a 99% tax rate, presumably it would be all the basic needs plus some more, but this would start to feel like a planned economy where individuals, with their small discretionary funds, would't have as much input into what is created in the economy, rather it'd be a course political process which could be slow to meet the needs of the citizens, or fraught with other unforeseen difficulties.