Internally, Google is not so different from an ecosystem of VC-funded startups. They try a lot of things. Some ideas work and others don't.
The alternative is to never ship new products out of fear of failure. Or, to indefinitely support everything, regardless of how tiny. Or, to wait so long to derisk a new product that they possibly miss out on a new market (I'm looking at you, HomePod).
At the end of the day, this strategy has worked out well for them, so why stop? For every failure like G+ or Allo, there's a success like Google Photos or Duo.
IMO if Google really wants to try out new things, they should make these things invite-only to find out if they are working and how users interact with it, or clearly mark them as 'beta' or something like that. Google has a huge fan base and the internet is full of people excited to try out new things. But if users clearly see "hey, this is kind of a beta product, I am just testing this out" there would be way less disappointment if the product shuts down after a while.
I might be biased but I think Amazon is like this.
Disclaimer - work in Alexa
For some definition of work. They're still mostly (~80-90%) an ad-driven company, after all these years.
Reality: after 2008, economy boomed, and advertising also did. Advertising, once you have the means, is easier to scale, and high margin. Google stopped relying on just owned and operated properties, and diversified into 3p properties to achieve even more scale
Most other businesses or business attempts require massive operations which google lacks
Even you ramp up non advertising revenue to a billion dollar, advertising will grow more just because it is already big.
One of these is unlike the other. The amount of money invested into the ultimately-failed G+ is mind-bogglingly immense.