Congress is only nominally a legislative branch at this point. We wait for a couple people to tell us what the law will be, and then we vote as a mere formality. Most members of Congress never read any of it and only vaguely know what they’re voting on.
— Justin Amash (@justinamash) September 30, 2020
A lot of things have contributed to this over the years, but there did used to be a time when at least to some degree there was a process of getting legislation through committees, then the full bodies, then a conference committee, then back to both for a final vote. Now basically everything is 2 or 3 way negotiation between the Leader, Speaker, and the White House. One contributing factor, though not the only one, is that the Leader and Speaker prefer it this way. And obviously that the White House has muscled in on the legislative task more and more appeals to them!