Most reasonable thinking people would understand your points. However, Democrats are vicious infighters and to them politics is blood sport. They are quite capable of being vindictive, even at the expense of severe political repercussions and self-inflicted wounds.
Since losing big in recent years what did they do? They lunged further to the left. I don’t want to even speculate on what they might do with Garland on the USSC, albeit for a temporary period.
On the whole, Obama has preferred to avoid causing those kinds of problems for himself unless a major objective can be obtained and made permanent. In this instance, I think that the marginal value of a single Supreme Court appointment would be far less than the enduring discredit and blame that would fall on Obama. I think that he would prefer to let Clinton's defeat linger in the mind of Democrats rather than have blame and anger against him damage his standing, influence, and earning potential.