why don't they all go with the free option by default

That question almost always goes to corporate support contracts. If I'm a big accounting firm and Excel breaks business-wide one day, I can get MS on the phone and fix it. I don't have to wait for some random guy in Eastern Europe who develops software as a hobby to get around to a patch when he has free time. Every second that my software isn't working is lost revenue. That's why enterprise paid for support contracts in the first place. Once a company is big enough, it only trusts the risk management style that's common with other big companies.

The free software you see get big corporate adoption is usually stuff in the programming sphere, where the company itself already has employees who can deal with it or another large company using it explicitly has people dedicated to supporting it. Like why OpenSSL has massive adoption and OpenOffice doesn't.