Because I don't have any fundamental problems with this.
For many families, that check is going to be a significant portion of their monthly budget. The pressure from "progressive" politicians to deliver more dollars to "working families" and to screw "the rich" will be applied to those checks.
Ah... but who are the rich and who are the poor? Without people filing 1040s on themselves, how's the government going to tell one from the other? While wage information is reported for Soical Security benefits, who's to say the person making $10,000 a year in wages isn't getting much, much more from non-wage income (property rentals, investment income, self-employment, etc.) -- there's no workable way to "screw the rich" and give to the poor without the IRS, which goes away.
Increasing the size of the checks would do two things: 1) raise the at-the-register (marginal) rate on everybody, rich and poor alike, and 2) increase the poverty line, which would give political opponents the opportunity to point out that the number of people in poverty increased under so-and-so's watch.