Some people do. Some don't. Since no winning lists are ever shared by POP players are fully aware that they have no responsibility to do so.
Generally those who don't share feel like others would gain an advantage if they did so. They don't want their opponents to know all of their deck's techs and whatnot. To them it’s about competitive advantage.
Others feel like they put all the effort into making the deck, why give that away for free? It would be like someone building a car and then letting anyone who wants to drive it at no charge.
Some go half way and give most of the contents out in their reports but don't post the exact list. Reading the report carefully will give you most of the deck but they want others to actually put effort in to make a build ... usually for one or both of the reasons above.
And finally some folks do share their lists. Some do it out of a desire to help the community, some do it to brag, and a few just feel like a certain deck is of no further use to them so they really don’t care what others do with it.
Personally I wish that all winning lists were published on the official site. One of the things I like least about the Pokemon community is the cult of secrecy. It makes perfect sense to me why it’s there now give that lists aren’t published but it really divides the community as a whole. You pretty much have to join a team or be friends with other good players to improve, since that’s the best way to get access to winning lists. The net result is a bunch of cliques. This debate seems to happen at least once a year. I don’t really expect change at this point.
Anyway if you really want to find the good deck lists what you do is:
a) Read the tournament reports. You can gain a lot from them even if there isn’t a full list in many cases.
b) Read the what won XYZ threads. This will give you a good idea of which pokemon are worth looking into more.
c) Read the deck help forums. There are a few winning decks posted there and a very few thoughtful people post very good lists there. Even the bad lists can sometimes give you new ideas. It will take time and effort to learn to distinguish who the people most worth listening to are here.
d) Read the other parts of the gym, especially the Front Page articles. Those are written especially to be accessible to new players these days. The card and news areas also frequently have good insight from folks.
e) Try to network with other players.