But the problem is that the 'moderate' factions of the GOP are still mostly quite extreme for the most part and agree with Trump on the vast majority of issues. McCain, Flake, Graham, Paul etc may diverge sometimes but most of the time they'll still happy to forward his agenda despite stated reservations, and they aren't fundamentally going to go against their party.
For left-wing Dems, there's very little to be agreed upon with the GOP right now. Their economic goals generally involve severe cuts/a reduction of the state (except when it suits them to do otherwise) and for the Dems, that's got to be something they oppose if they hope to regain power again. The approach you're advocating is one I'd argue they've largely tried until now, and for the most part it's only gotten them a whole host of defeats. There will be rare cases when they can agree with - and work with - the GOP, but on the vast majority of issues they should be staunchly opposing them. Otherwise they aren't going to win over voters.
They may be extreme to you but they are fairly centrist in terms of the US political medium. Left wing dems have no power right now other than shouting and obstruction. Instead of blindly doing that across the board they should find ways to actually get policy through. Infrastructure is a good example - Trump is for it and it creates loads of jobs which both parties can agree on.