why do tech companies hire so many contractors despite at-will employment

I'm from Europe and have been living in the US for a couple of years now. I have always wondered why there is so much contracting jobs in the tech industry even though there is at-will employment which makes sure a company can let employees go for any reason (PLUS considering a contractor is quite expensive since the contracting company earns a good portion too!) . Coming from Europe, I'm familiar with the concept for contract jobs, however, the situation is different there as for most parts in Europe there is no such thing as at-will employment which means if companies want to be flexible in terms of workforce their only choice is to hire contractors. But this is not the case in the US... someone please explain this to me...

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