Unlike regular indoor furniture, patio furniture not only has to look great but it also has to be able to withstand being exposed to the elements. Changes in temperature can cause materials to expand and contract, so outdoor tables and chairs need to be designed and treated so that they will not crack or become less stable from season to season.
Cheap or untreated materials are more susceptible to water damage, while high-quality hardwoods and metals with powder-coasting are much better-suited to last and avoid rot or rust. Similarly, low-end fabrics will fade in the sun, while high-tech solution-dyed textiles are durable and colorfast. Inexpensive patio furniture may look great for a while but you'll likely have to replace it every year or so.
Luxury outdoor furniture may cost more initially, but it will look great for many seasons and save you money in the long run.