Because the number of confirmed cases in the US is increasing at a faster rate than any other country on Earth, and it's only a matter of time until the systems get overloaded. The main reason Spain and Italy are seeing such alarming death rates compared to the rest of the world is that they don't have the capacity to treat all their patients. The US is on their way to that point as if it's a fecking race to get there first, and as soon as it happens the shit will really hit the fan. It's more than likely that the number of Americans dying every day will reach five figures well before the end of the month. Donnie's out of his element if he thinks it'll stop at 240,000 total.
But that is to a large degree because of the number of testings going on. The US is testing daily (more or less) as many people as the UK has tested in total since this shitshow started. If you test over 100k people daily, of course, that the number of infected people is going to increase. We saw the same in Europe when Germany looked relatively not attacked, they raised their testing and now are fourth in the world in the number of cases (they will probably surpass Italy in a couple of weeks).
When the number of testings is very different from country to country, it is pointless comparing the number of infected. The number of deaths is more relevant, and well the US numbers are increasing there but are nowhere as bad as Italy. Italy was having almost 1k deaths per day a few days ago, adjusted for size, the US need to have 5k+ deaths per day. Spain is having 1k deaths today, adjusted for size, the US needs to have around 7k deaths per day to do as bad as Spain.
It might happen, or not, but I haven't seen a pattern so far to suggest that they are doing worse than Spain, Italy, the UK, or France. The UK, for example, has 1/5 of the population, while having 1/2 of total deaths and daily deaths. And has issued the lockdown later than most of the US states, while has a far weaker medical system (number of ICUs and ventilators).
Germany is doing unquestionably better though (so far). Is it just pure luck or they are just behind and inevitably it is going to get there, I don't know.
At the end of the day, there has hardly been a vast difference in how the large Western countries have acted, so I don't think we will see a large difference in the number of deaths when all this is done.
Finally, only New York, New Jersey and to a lesser degree Louisiana and Michigan have the medical system near collapse as Italy and Spain. In others, the medical system is still quite strong. And some of the states like Cali or Washington have been attacked earlier, but the measures the governments took seem to have had a large effect. Additionally, the US can mobilize better than most of the other countries (just see the work in New York going on, and compare it with Lombardy) in building new hospitals or making new ventilators.