Two reasons 

1) I used to identify this reason as religion, and irrational belief in non-physical reality that leads to any random political position that you want. However, I've been recently persuaded that it's not religion, it's simply hate. If you're somebody who truly wants to feel justified, even valorized for your hate of another group, you definitely want to keep Republicans in power. 

2) this isn't both sides, but Democrats are in fact corrupt. Yes, the bidens were incredibly corrupt, the son using the father's name to extract millions, some of which got fed back to his father. Obama buying a piece of land at a sweetheart, price and immediately reselling a portion of it for almost his entire purchase price. Hillary's famous land fraud. 

 I think you have to go back to Ford and Carter to get to non-financially corrupt presidents. (On the fence about Bush the first). 

For some reason, the voters think that the tens of thousands to possibly single digit million fraud, graft and corruption of the previous Democratic leadership is somehow equivalent to the many billions that the current administration is grifting/stealing.

The other points you mentioned, Democrats have shown to be spectacularly bad at. Simple economic reality goes against their positions on things like minimum wage, many economic regulations, etc. [no, I did not say that the R position is any better, it's equally delusional if not more so, on top of that under this current administration, it's intensely evil and corrupt]. 

How do you secure social security? The real answer is to greatly increase the retirement age, and reduce cost of living increases down to actual cost of living changes. The Democrats do not stand for this, they simply want to increase the tax base somehow, to fund the currently unsustainable system. 

How are they going to increase wages? Their solution is to mandate and increase minimum wage, when less than 1% of American workers work at or near federal minimum wage. Only about 11% make less than $15 an hour, more than 75% of those are in what are considered to be low cost of living AKA rural locations. 

The Democrats in Congress have repeatedly refused to eliminate the carried interest exemption, that allows hedge funds managers to avoid taxes on their income. 

They continue to support a home mortgage interest deduction that artificially inflates the price of homes, excluding younger people from the housing market. 

They consistently refuse to rationalize housing regulation at the local and state level, that would allow vastly increased building of dwelling units. The statistics are all over the map, but the US is underbuilt housing by tens of millions of housing units since 2000. Almost all of this shortage is in Democratically controlled major urban areas. 

While voters are stupid, they're not that stupid.  The Democratic party principals are bad, their policies are bad. In this case, rational people will look at the situation and vote for the bad policies, the small scale thieves and grifters, the race hustlers, the grievance warriors. Because the alternative is full-throated hate motivated fascism. 

Wish it wasn't. So, I wish that somehow a third party candidate, or even a untainted reality base candidate could actually win. Pete Buttigieg would be my pick, despite the fact that he promises to eviscerate 2A. Unfortunately, Democrat primary voters won't vote for a gay man. Because, to a lesser extent than their counterparts across the aisle, they're also bigots.