People want Trump to concede not because they’re bitter, but because they want him to behave like a civilized adult. Needless to say, they’re bound to be disappointed.
You're not 100% accurate here.
A huge part of the narrative in the leadup to the election, during the election, and after the election, has been focused on how the left wanted this election to be a clear "referendum" against Trump. If you doubt this, browse through media coverage and prominently displayed opinion pieces on sites like WaPo and NYT - I won't post specific examples because there are quite a few and they're easy to find. But that was the desire - to have an election that was such a humiliating, decisive loss for Trump that they would get to circlejerk for the next 4 years over Orange Man but in the context of "Orange Man LOSE!"
What they got instead was a Pyrrhic victory. Trump gained voters among the minority groups everyone swore wanted his head on a plate, he gained millions of votes compared to 2016, he got his SCOTUS picks in before the election, and the GOP is still set to control the Senate, aka the body of Congress that actually matters. To add a piss filled cherry to the shit sundae they now have over 70 million people pissed off about the results and quite a few of those people believe the election was outright stolen from Trump in a coup attempt. Whether or not that bit is true is tbd but it doesn't exactly shape up to a positive outlook over the next 4 years if you're a Democrat.
So they need Trump to concede to try and salvage any of this situation but he is refusing and it's amping up his base even more and giving more fuel to the fire. And that absolutely pisses them the fuck off and makes them bitter because this was supposed to go so much more smoothly for them. They're bitter because of what the results symbolize - that despite their best efforts, Orange Man still became even more powerful and gained even more votes.
You might be speaking from your own experience when you talk about wanting him to behave like a civilized adult. Good on you. My experience however, which has involved reading bullshit from both sides for the last 4 years in the leadup to this election year, has shown me that there aren't many civilized adults on either side really. And while the right tends to look down and pass judgment from a religious/moral stance, the left 100% also loves to assume the high road in their minds and pretend they are the shining city on the hill, looking down at all of us unwashed masses in disgust. tldr it's not so much that most of them want or even care about him acting like a civilized adult and it's disingenuous to even attempt to say so. If Trump had conceded on day one there would still be a very large portion of the political left who would still mock his decision to do so, only instead of "what a baby, Trump won't even concede" it would be "holy shit what a baby Trump conceded".