The US system has the senate, congress, presidency, and a bunch of committees - all of which can impede the others' work, and all of which can be under the control of a different party.
And every person who can block your bill is someone who can extract a concession. You want your bill to pass, and for a mere extra 0.3% pork you'll have my enthusiastic support....
With the result they get thousand-page bills, and a country dotted with more dams and bridges than they can afford to maintain.