Why do people think Bernie would have done worse than Biden?

[EJ24]

Because Bernie Sanders was never a strong candidate, even in the context of the Democratic primary.

He benefitted in 2016 from a small field and a very unpopular frontrunner named Hillary Clinton. Even then, he lost by a pretty significant delegate margin, and struggled with core voting blocs like older voters and African Americans.

Then in 2020, he first benefitted from a large field splitting the moderate vote while he hogged the progressive lane. When these moderates coalesced around Biden on Super Tuesday, Sanders got trounced. He once again struggled with the same voting blocs of 2016, but this time did much worse with rural whites, indicating that much of his support in 2016 was anti-Clinton and not pro-Bernie.

You want to nominate a strong candidate for the General Election, and all the evidence indicates Bernie was incredibly weak.

[Schiff for Senate]

Sanders would have likely alienate the suburban moderates who were the backbone of Biden's victory in Arizona, and probably Georgia, too (GA was a mix of huge turnout in Atlanta and suburban support for Biden). And not just suburban moderates - moderates generally might have stayed home or supported a third-party candidate over Sanders (a diehard progressive) and Trump (a controversial Republican). Moderates are a key voting bloc for Democrats, and 2020 was no different - per exit polls, self-identifying liberals constituted just 24% of the population, and broke for Biden 89-10%, but self-identifying moderates made up 38% of the vote, and they too went for Biden by a large (albeit smaller) margin: 64-34%. If Sanders was the nominee, he may have done even better among liberals, and he would have boosted turnout among progressives as well - let's say with Bernie on the ballot liberals now make up 26% of voters and they go for him 91-8%. But many moderates would likely go for a third-party candidate or even Trump (determining that Trump getting reelected is an all right cost for not having a progressive in the White House): they would, let's say, now make up 37% (instead of 38%) of the vote, but they'd go to Sanders by a much smaller margin - probably around 56-37% or so. The huge loss of moderates (who significantly outnumber liberals) would cancel out any gains Sanders may make among moderates - and then some.

[David Hume]

It's highly unlikely for Bernie to win AZ and GA. He may have some popular appeal in the rust belt, but huge turn off to wealthy suburbs as well. One more state lost means losing white house.