My guitar does the same thing and I also leave it out. Having it out exposes it to the elements. Even the air around the room can affect it, especially after a long play session where sweat gets on the strings. Putting it in a case helps, but makes it inconvenient to pick up and play.

Strings aren't meant to last a long time. Replacing them every 2 weeks if you play a lot is not uncommon. I change mine about once a month and I play with dirty strings most of the month.