Biden took office in the middle of a pandemic. It was probably a good point for the Dems to use for economic recovery comparison. Comparisons should be done with the economy before the shut downs began.
This is similar to the global warming hoax which used the mid 70’s as the starting point without stating that was when the climate analysts said that we were entering a mini-ice age.
I think the lockdowns were planned so democrats could say how bad the economy was, and take credit for the recovery.