Because a shit ton of our GDP is generated by federal spending. They hire contractors, they buy shit, they build shit. When people talk about cutting government spending what do you actually think that means? It's not just cutting a bunch of dead-weight federal employees (and yes there are a lot of them).
Does that mean I think we can sustain our current spending levels? Fuck no, but we have a problem that needs a gradual reduction over time to avoid "shocking" the economy. We got ourselves into this mess over a long period, it's going to take a long time to get out of it - there won't be any quick-fix solutions. This is the main reason I trust Johnson over Trump, his temperament seems less erratic and he actually has experience governing. Trump's good for what? Swindling people in his sham of a university and declaring bankruptcy a bunch of times. No thanks.