I’m just thinking out loud here, but if you made it out of exterior grade plywood and then, after milling, gave each piece a thorough coating (several coats) of marine grade varnish (ask at your local hardware store) it might last a few years outside. (This is the kind of varnish used to protect wooden boats.)

Another factor is that if you place the structure outside without a roof in a rainy environment the plants are likely to be drowned since there’s no drainage built into the structure. If you were to use it as an outside structure you would probably need to add drainage holes. I suppose that is possible, even with a plastic liner, as long as the edges of the holes are thoroughly sealed and the liner is pierced to match the drainage holes, but the whole thing seems a bit problematic. Not impossible, just likely to have a quite limited lifespan. Even with a good coat of sealant, constant contact with soil and moisture will eventually work it’s way through the sealant and start to rot the plywood.