Because how could the Democrats be certain that the Republicans wouldn't just use the same accounting trick again the next time that they wanted to pass tax cuts via reconciliation (which would effectively let them double count the "savings"). The Republicans have shown very little good faith in bipartisan efforts over the last decade and a half (as just one example, all the R's who voted against the IRA only to then campaign on the achievements of the bill later in the year).
So while it is great that the Republicans fixed this one thing (that they themselves broke), asking why the Democrats didn't fix it kinda feels like you have been living under a rock for these last few years. If they break something, and they regret breaking something, let them expend the political capital to get it fixed. There is no free lunch in politics. If you spend your time on something like this, it means some other priority is getting ignored. And doubly so if your counter party has been operating on such bad faith as of late.
The democrats were elected to undo the damage caused in Trump's first term. And they let the damage start and happen on their watch, because.. according to you, for the sake of political benefit while ordinary people suffered and are still suffering from losing and not getting jobs. They will probably do the same again if they're elected, intentionally keep things broken for political gains. It's sad to see people support and encourage such behavior.
So while it is great that the Republicans fixed this one thing (that they themselves broke), asking why the Democrats didn't fix it kinda feels like you have been living under a rock for these last few years. If they break something, and they regret breaking something, let them expend the political capital to get it fixed. There is no free lunch in politics. If you spend your time on something like this, it means some other priority is getting ignored. And doubly so if your counter party has been operating on such bad faith as of late.