Go was invented by the Bell Labs contingent, not people who care about Google's products.
And C++ is called "cpp". It's a non-issue, not a problem. And it fits the Go ethos of not overcomplicating the whole language to address minor concerns.
But that isn't for searchability but due to + not being permitted in names in some filesystems, or being rejected by some tools even where they are valid in filenames in the current environment, so file.c++ wasn't a good idea.
The ability to find things in a global search engines was not the concern that it can be today back in the early 80s when C++ was named, so this is not really a like-for-like comparison, and anyway a quick test in common search tools shows that searching for C++ seems to work well enough.
And C++ is called "cpp". It's a non-issue, not a problem. And it fits the Go ethos of not overcomplicating the whole language to address minor concerns.