He didn't step aside because incumbency is a huge advantage. Harris is viewed worse than Biden is, with approval ratings in the 37% range (below Biden's 38%). So replacing Biden with Harris removes the Incumbency Advantage without improving any other metrics. What else do we have?

• Bernie Sanders: He's lost 2 times already in the Primaries and has an approval rating in the mid 40s. He's also 82 years old.
• Elizabeth Warren: She's got an approval rating in the low 40s, and is 74, and a woman as well.
• Michelle Obama: While she has a mid 50s approval rating, she's still a woman, Black, and related to a former President, all things we've seen run against you. She's also not been tested politically yet. She's at what I consider the optimal age to be Potus, at 59, but given she's never run for office before, she has a huge liability I expect to be exploited if she did run. And let's be clear. She doesn't want to. Made it clear repeatedly that she doesn't want to run for office and is happy working behind the scenes without being a politician.
• Alexandria Ocasio-Cortez: A potential Progressive Firebrand youngster that some people would scream with joy at being able to vote for, but even more would vote against her. Mid to upper 30s approval ratings make it clear she's NOT a desired candidate for most Americans, with a good 63% saying "No, she's not a good politician." Also, this is the first year she could run as POTUS (she'll be 35 years old in October of 2024). Her experience is limited to her Congressional District, and she's not run in a wider election. I personally think she needs more time in the oven, running for a State Office such as Governor, Senator, or so on. Regardless, getting one of the Squad into the Oval Office sounds like a tough sell for America, to me.
• Gavin Newsom: You'd think the governor of California would be a logical first choice for President. Thing is, he's polling in the mid 40s approval rating in his own state, and 7 in 10 reject his running for POTUS, which would make his job in Sacramento insecure if he tried to have a go at Washington. And this is before we talk about how much California is reviled in parts of the US. I can't find national polling numbers, but if he's only polling in the mid 40s in California, I can only imagine his numbers nation-wide, and that's BEFORE the Republican attack ads. Don't forget he has some things he's done that can be used to generate attack ads from the Left too!
• Robert F. Kenney Jr: This guy is a nutcase, a virtual MAGA in Donkey clothing, with downright dangerous views on vaccinations. He is in the upper 30s in terms of approval rating, and isn't even running as a Democrat, but instead, as an independent. He's seen the writing on the wall, and knows that Biden's incumbency advantage will be too much in the Dem primary, so he's given up on that. He might poach enough votes from the Dems to elect Trump in 2024, but current polling suggests he's more a threat to Trump than Biden (as usual, take those polls with the usual caveats and turn out to vote for your chosen candidate -- the only poll that matters is the one held a year from now). Side note. Here's a "fun" read on the guy. A whackjob for sure, but one that indicates we shouldn't ignore the siren call of conspiracy theory bullshit on our side. After all, we are the original home to antivaxxers before it became fashionable on Team Red to reject basic biological science because "ain't nobody gonna tell me what to do!"
• Marianne Williamson: Who? This 71 year old nobody was -4 points underwater in 2019 in approval rating, but the real numbers were that 53% of people didn't even know who she was or had no opinion of her. She's polling at below 10%, even after RFK dropped out. The latest head to head poll I can see has Biden taking 73% while Williamson takes...5%. 'Who?' indeed...
• Dean Philips: Another nobody who joined the party late. Like Williamson, he's polling at below 10%.

Biden is the Incumbent. He has a proven track record, name recognition, and the backing of the Democratic Party. He's beaten Trump once. He's the most likely to beat Trump again. Switching horses mid-race has never been a good idea, in horse racing OR politics. Just look at Carter for a good example of that. Unless there is a compelling reason to swap Biden out (such as death of the candidate, conviction of a major crime, involvement in a major scandal), as the old saying goes, in times of trouble, go with who you know.

I do think Biden could use to swap Harris out for a more progressive firebrand, or at least somebody younger, and this time make the VP be more front and centre. When confronted by it, he should say, "Hey look, I'm just an old man. I'll be 86 years old when I'm forced to retire by the 22nd Amendment, so I'm going to focus on making things work, and (s)he can focus on the PR crap." Just for shits and giggles and to spike the cannons of the 'he's too old!!!' crowd...and to set up his VP to deal with a Don Trump (third or first for jr) attempt to take the Whitehouse in 2028.

Defeating the shithead again in 2024 won't make him go away. I'm sure the oligarchs on the GOP side are planning for their next tin-pot dictator. Putin and Xi won't be denied, after all.