The iPhone was released in 2007, but a smartphone that could technically have called a ride predates it by at least a few years. Uber was founded in 2009, but actually release the product and app until 2011, after a beta in 2010 [0]. It didn't hit a $1B valuation until somewhere between February 2012 and August 2013 [1]. So 4 years until the emergence of the X in "smartphones led to X", 6 years until it was clear that X was a $1B opportunity, so unless you're completely aware of every product spawned by Alexa or voice assistants generally, I don't think this is a fair comparison.
Even saying that, Alexa and competitors don't need to be computing platforms, they just need to be the interaction layer for day-to-day human-computer interaction. Even if Alexa were as technically simple as an IFFTT recipe with the "THIS" being "what the person says out loud", that's hugely valuable. It's very clear to me that if it's easier to do something with on device than without that device, and even easier to make a device do a thing without physically touching the device, that will ultimately be the way most people do the thing, provided they have access to the technology.
That is why Amazon is pumping out so many different Alexa form factors. I couldn't figure out who would want the Alexa ring. Then I met someone who hates ear buds and swears by his Swiss watch who was excited about a wearable he'd use. Amazon wants to be the dominant player, and is simply crossing form factors off of their (presumably) stack ranked by opportunity size list.
[0] https://en.wikipedia.org/wiki/Uber#History
[1] https://pitchbook.com/news/articles/uber-by-the-numbers-a-ti...