Because there are only two options. Think of aggregate voter sentiment as being "If things are good/ok, vote incumbent. If things are bad, vote for change/the other guys". It doesn't matter who the 'other guys' are, just that they're recognised as 'the only alternative'.

Eg. Canada's third largest province just elected a totally unknown conservative leader -- like the 'real' leader stepped down a day before the election was called, and a temp got put in place... who then 'won' the election. That cycle wasn't about "voting for the random temp" / supporting conservative values, it was a repudiation of the previous government. People just didn't feel like there was any other viable alternative.

So if the dems don't markedly improve the living conditions in the USA somehow during their term, and get people thinking "Yeah, things are good!", they'll be kicked out again, and replaced by whatever monstrosity the R's have dredged out of the Trump Ocean.

Be prepared. Media outlets will memory hole the whole thing.

they always end up starting wars to ensures any DEM will inherit the mess, the voters, mostly conservatives and some independents will have goldfish memories and vote for a republican again.

They time their legislation to kick in during Democratic administrations, then blame Democrats. The morons fall for it and there are a lot of morons in this country. We need Democrats calling out shit like this all the time, teach people about these strategies.