There's also the fact that the Democrats are much less of a monolith than the Republicans.
Susan Collins and co can say they're allowed to deviate or have a different opinion, but all in all, the Republicans tend to be one big block in politics, as far as voting is concerned. The breadth of the spectrum is also smaller than with the Dems (and than it used to be).
I can just about imagine the Dems gaining complete control over WH, House and Senate, only to spend the next 2 years bickering amongst one another. After all, they're the party that tries to encompass everything from the extreme left fringe all the way to the moderate conservative slightly right of center. Say, Biden and AOC, simply have little in common on how they want to solve issues - though they can probably at least somewhat agree on what the problems are, which the Reps wouldn't agree with.
If Biden was the Anti-Trump and the Democrats voted as one huge block on everything, he could add 4 SC Justices, expand MedicAid to cover anyone who wants to, raise the federal minimum wage to $15, reconnect with Europe and the UN, launch a Green New Deal, tax the rich, dial back the corporations-as-people and money-is-free-speech crap, etc etc. That program would get you maybe 20% of the vote, though, and at least half the Dems in house/Senate would vote against it.
Slow attempts to find common ground, rebuild fractured society, halfway attempts at ficing things, expect nothing else.