I accidentally stumbled upon this 'trick' a week ago when I forgot about a baguette that I bought for a party and never used. It was half-covered (loosely) by a plastic grocery bag. When I took it out after the week period, the top half (not covered by plastic) was rock hard, but the part inside the plastic bag was still relatively soft.
What biological mechanics are causing plastic grocery bags to keep bread fresh longer? This Q/A discusses storage methods, of which plastic bags are one, but doesn't actually explain why it works the way it does.
I also stumbled up this video where a guy mentions that he also keeps bread in plastic bags, so this isn't an isolated incident.