So, I'm not getting it. Funneling even more money into the government, where it has been proven to be wasted, is somehow better?
Well it's certainly wasted where it is now. Hoarding the money in tax havens and other such things essentially makes it useless to the economy. You want it to circulate through the economy to keep it working. Large spending programs are a way to do that (science and tech, education, welfare, infrastructure etc) by creation of jobs.
The other option would be considerably increased wages across the board and a vast increase in the minimum wage - the average wage for the middle class hasn't really moved in decades (proportionally to the cost of living), unlike the compensation for the super wealthy, which has skyrocketed. Great for them, but at the cost of stagnating the economy.
There's no simple answer, but low taxes on the wealthy is certainly not among the right options. It just siphons money away from where it does any good. It's like reducing the amount of blood you circulate in your body, keeping a large volume of it in a separate body that you can't use.