Some 'one things' scale to epic proportions, like Google's search engine or Facebook, and become large stand-alone products all to themselves (and dominate their category).

If your product doesn't have that capacity, you're likely to eventually end up as the target of a bigger company that has that. The Windows ecosystem was famous for that, Microsoft continually ate things that were useful and put them into Windows, killing off entire niche product segments.

The iPhone is about 3/4 of Apple's entire business (the iPad shrank significantly this past quarter, and nobody cared much, because of the iPhone's growth). Search + ads is the extremely share of Google's business. And Facebook (11 years old) only makes money from its core social network for now.

Most very successful companies only do one thing really really well. There are a few conglomerates running around, but they're a very small fraction of businesses.

Also, being good primarily at one thing, doesn't mean you can only ever be good at just that thing. The iPod has all but disappeared as a meaningful business for Apple, to the extent that they essentially no longer talk about it. Apple has been good at one thing, with three different unique things: PC, iPod, iPhone.