It's more like a slope, actually, but the two big ones are:
1. The special US$600/week unemployment benefit top ups only currently apply to unemployment occurring through the end of July, 2020. (Actual claims are paid in arrears, so a good chunk of the US$600/week top ups will be paid out in August and even beyond.) Starting with unemployment experienced from August 1 onward, standard state level unemployment benefits and benefit durations will apply under current law, and those standard benefits are probably not even good enough during periods of low unemployment.
2. State and local governments either have set their budgets or very soon will for the next fiscal year. Almost all of them are required to maintain balanced budgets. Their tax revenues have collapsed due to COVID-19 impacts, and their relief spending is higher for the same reason. So they're already well underway with massive workforce layoffs and other spending reductions. State and local government spending in 2019 represented about 17% of U.S. GDP, so as they balance their budgets the whole U.S. economy suffers. The only entity that can prevent this from happening, or at least make it less bad, is the U.S. federal government.
The Republican Party controlled Senate is blocking funding for both of these urgent problems, not "Congress."