It's not that you can't do some of it indoors. Composing a poem can take place anywhere. Most kinds of meditation work just as well indoors as out. My altars are inside my apartment and I live a primarily urban life.
Yet some things just work better outside. I find I contact the land spirits and many of the deities better when I'm away from buildings and pavement and noise. Being surrounded by trees or walking the beach speaks to me differently than sitting on a couch or lying on the floor. Intensive meditative ritual flows better for me when the only sounds around are wind and water and the songs of birds.
When I was growing up in rural western Massachusetts, I'd regularly spend all day outdoors, sometimes even in the winter with the snow hip-deep in the woods. I felt more comfortable with the local forest than with most people. I watched birds and animals, learning where they lived and what they ate and what their tracks looked like. I learned stillness and patience so that they'd come close.
Living in the city, I still need to get outside. My most intense experiences are multi-day ritual work while I'm camping and immersed in the natural world around me. For a lot of people in urban North America, opportunities to get out of the city are rare, but I believe they're essential to the practice of filidecht. Without being in the flow of the seasons, without being away from people and surrounded by trees or desert or water, without having an opportunity to observe non-human life, I think that filidecht is just a sterile exercise in poetics.
Life enclosed within the walls of a house or an office is circumscribed and restricted. While filidecht can be practiced solely within the confines of a city or in urban parks, it finds its wealth and depth in natural places away from human influence. I think it's particularly important for those of us who are working through the issues of being geilt to be able to get out into the wild as a clearing and meditative practice. This doesn't mean rejecting the human or the urban so much as it means embracing the Other and learning to see yourself as existing within that world.
Human culture carries an incredible amount of pressure within it and I feel like it's easier to clear my head and listen when I'm away from my urban environment. Staying indoors might be tempting for a lot of people, but I believe that a practice that involves work in wilderness is more effective than one that takes place entirely indoors and in urban environments.