Calls were made too early too many times over the course of the last couple of months, eroding the trust of the 99% of the completely normal Trump supporters out there.
Even if the Democrats are 100% sure the election is totally squeaky clean there should be no problem to go over it with a fine tooth comb so that everyone can be certain. Instead any attempt at transparency was ridiculed from the beginning with lots of goal post moving to follow about just how much discrepancies there actually were.
Some law changes right before the election also don’t really make sense (like waiving the need to verify signatures and in fact making it impossible to do after the fact by destroying envelopes) and instead if bringing clarity to the situation, mainstream media ridiculed anyone who had doubts about this as being crazy.