When the economy is growing, investment makes sense. Why put your money under the mattress when it could be out there, working for you?
When the reverse happens, investment stops making sense. Why risk your money when it becomes worth more while it is sitting under your mattress?
But stopping investment does not just mean stopping speculative investment. It means stopping investment in other things as well. Like maintenance. This guarantees that things are going to become worse over time. Which is a feedback loop that makes investment even less worthwhile.
This has happened in the USA before. The last time is called the Great Depression. Read through accounts of what it was like. Would you like to go through that now?
History also teaches that the longer it is between economic setbacks, the worse the next one tends to be. We've gone far longer since a depression than at any point in history. Our next one is likely to be correspondingly more terrible.