I also find it interesting that you also compared them to Microsoft, since Microsoft is now opening retail stores.
The key reason Apple isn't considered a monopoly is that even though they are huge, they don't' have a monopolistic position in their products. For smartphones they have a market share of ~20% worldwide and in the PC market the hold ~5%. Even on the country level, the numbers never get to a level where people would usually scream monopoly.
You could argue that they probably have monopolistic market share on the tablet market but most wouldn't consider that an industry in itself (yet) and they haven't exhibited the characteristics of a monopoly: price setting for the industry and an inability for new players to enter the market. It's actually possible to not be considered a monopoly even with a huge market share if you can prove you don't control the industry.
Apple's normal business model allows them to thrive without being a high volume business; their per device profit margin greatly exceed any competitor, so they can still be hugely profitable with low market share.