IKEA has not been copied because it is hard! They have a strategy that is very difficult to replicate. In his seminal work on business strategy, Michael porter says that the essence of strategy is choosing what not to do i.e. picking a differentiated set of activities that your business will undertake. You can think of it as picking a distinct combination of elements that result in a particular outcome.
It is NOT doing the same things better! That is operational effectiveness, which is different from strategy:
"Operational effectiveness (OE) means performing similar activities better than rivals. OE includes but is not limited to efficiency. It refers to any number of practices that allow a company to better utilize its inputs by, for example, reducing defects in products or developing better products faster. In contrast, strategic positioning means performing different activities from rivals or performing similar activities in different ways." [source, Harvard Business Review article on Michael Porter's work: see PDF below]
He gives the example of both IKEA and Southwest airlines. Both picked a differentiated set of activities borne out necessity. For IKEA it is:
"out of the way" location of their warehouses where real estate is relatively cheap
Flat pack furniture that takes up less space both in shipping and storage at their location (so they gain two big advantages from this.
Getting customers to 'self serve' in activities that would usually require paid staff i) Having a sales rep show customers furniture in the store ii) Assembling the furniture.
Elegant Scandinavian design.
It takes years to perfect each one of these. Porter uses a simple mathematical analogy to explain why it is so hard to copy. Assume each of the four activities has a score of 1, and to have success you must multiply them i.e. 1 x 1 x 1 x 1 = 1.
If a competitor attempts to copy, chances are they won't get a 1 across the board the first time. They could get 0.9 on Scandinavian design, 0.8 on 'self-serve', 0.85 on flat pack and 0.77 on the real estate strategy. Multiply these and you get 0.9 x 0.8 x .0.85 x 0.77 = 0.46! Anything less than 1 is failure.
The more critical activities there are to a strategy, the more difficult it is to copy. Even though we like to simplify and boil down things to their essence, there is rarely "the one thing" that contributes to long term business success. The corollary of this is, If there is only one thing, you are much more likely to be copied and soon your advantage will erode. Unless that one thing is something patented like Google's PageRank (the reason why Google is today worth north of$700 billion) or the Bessemer process for converting iron into steel (responsible for much of Andrew Carnegie's fortune).
Below is an excerpt on IKEA from the Harvard Business review article:
"IKEA, the global furniture retailer based in Sweden, also has a clear strategic positioning. IKEA targets young furniture buyers who want style at low cost. What turns this marketing concept into a strategic positioning is the tailored set of activities that make it work. Like Southwest [airlines], IKEA has chosen to perform activities differently from its rivals.
Consider the typical furniture store. Showrooms display samples of merchandise. One area might contain 25 sofas; another will display five dining tables. But those items represent only a fraction of the choices available to customers. Dozens of books displaying displaying fabric swatches or wood samples or alternate styles offer customers thousands of product varieties to choose from. Salespeople often escort customers through the store, answering questions and helping them navigate this maze of choices.
Once a customer makes a selection, the order is relayed to a third-party manufacturer. With luck, the furniture will be delivered to the customer's home within six to eight weeks. This is a value chain that maximizes customization and service but does so at high cost.
In contrast, IKEA serves customers who are happy to trade off service for cost. Instead of having a sales associate trail customers around the store, IKEA uses a self service model based on clear, in-store displays. Rather than rely solely on third-party manufacturers, IKEA designs its own low-cost, modular, ready-to-assemble furniture to fit its positioning. In huge stores, IKEA displays every product it sells in room-like settings, so customers don't need a decorator to help them imagine how to put the pieces together.
Adjacent to the furnished showrooms is a warehouse section with the products in boxes on pallets. Customers are expected to do their own pick-up and delivery and IKEA will even sell you a roof rack for your car that you can return for a refund on your next visit"
Here is a PDF that summarizes Porter's work on Strategy: