Is it cheaper to cut down a forest than to buy a agriculture field?

That area was planted to be cut down. It was a monoculture for wood harvesting and the surrounding area still is, AFAIK. The only change to the original plan has been to not plant young trees again after cutting everything down and instead build a factory.

Effectively, they did buy an agriculture field.

Depends. If you need specific location for logistic reasons, in the long run it might be

Probably since physical labor jobs is valued so cheap and thats not mentioning that immigrants are easily used for that kind of work for even cheaper