What prevents you from going freelance and working on shorter contracts?

Lack of benefits, job security/confidence you will find 6 out of 12 months of contracts on a regular schedule. You'd need to be pretty sure you could average out 50% of your pay (more like 60-70% to cover benefits and the extra taxes) over 12 month periods and have enough buffer to cover unexpected gaps. You also have to account for the unpaid time 'working' to find those contracts and the contracts may not be the same pay rate each time. Its doable, but its certainly not as straightforward as it seems on paper.