Early Google initiatives also had a high failure rate. (Buzz, for example.)
My guess is that there are still Googlers trying to improve the web. Young people are idealistic, so why wouldn't they? But nowadays it's unlikely to be successful unless it's relatively uncontroversial infrastructure. (Some examples might be things like certificate transparency and QUIC, which became HTTP/3.)
Higher-profile initiatives to really change things often fail because they raise deep suspicion and resistance. They're certain to be misinterpreted in the worst possible way.
Also, significant changes affect vested interests. Some of those vested interests are internal.
To my recollection, G+ was actually pretty good at launch. It just was killed (or hobbled, for future killing) incredibly early for a network-effects, non-first mover product.
When it was a disparate group of products... incentives were generally aligned with the users of those products.
When they began to look at themselves as a platform company (Google search-on-everything, Android, Chrome), that fundamentally broke and they started making sound-platform-business but user-hostile decisions.
So I guess the moral of that story is that platforms will make you rich, but you have to be very careful to enunciate your value priorities clearly to users. (E.g. Apple: "privacy"; Google: "openness"?)
Google killed the king, and the king was the desktop. The king was apps. Microsoft seemed omnipotent & in total control, and the rise of the web isn't totally Google's but they sure did a lot and they sure rode that wave.
My personal feeling is that Google lost the ball in the g+ era. Up until then, it felt like Google understood their role was to help others create value, that they had to offer APIs and platforms to let other developers onto the platform, let other people expand the value proposition. G+ was an about face, a totalizing product push, and one that offered nothing to the world. Essentially no API offered. Google wanted to make g+, they wanted to run itz and if you wanted to use it, you needed to use their client and your account with them.
Where-as in the past, with efforts like Buzz, they we're trying to expand the protocols & value of the web as a whole. Once they gave up on platform & tried to be a product company, it was much harder to believe in the futures they were trying to sell.