Even if it had evolved, any detailed form of communication that had the potential to transmit hard-to-break imperatives is something you want to be very, very careful with.
Defection, manipulation, and novel avenues for disease-transmission or parasitism heavily disincentivize this. It's intuitively "gross" for a reason.
TL;DR: Infections and defections would probably utterly wreck this. The blood-brain barrier exists for a reason. While we did get language, in other ways we've evolved specifically to hide information from each other; it's not straightforwardly evolutionarily favored. Large clusters of highly-related organisms have more incentive to do this (bacterial mats, ants, our own cells, etc.), and the information-bandwidth they share with each other through pheromones and chemical signals is actually pretty staggering. But at a glance, I do think they pay a cost in increased (and more elaborate) avenues for manipulations and infections to reap the benefits of this privilege.
Edit to add: Linking some additional strongly-related articles! SSC's Maybe Your Zoloft Stopped Working Because a Liver Fluke Tried To Turn Your Nth-Great-Grandmother into a Zombie and the paper it centers on, Invisible Designers: Brain Evolution Through the Lens of Parasite Manipulation
Isolating Brains from Infection
It bears pointing out that evolution usually seems to want the opposite of this for our centralized-decision-maker organs. Notice that instead of making the brain more open-access over time, most species went the direction of making it isolated from even our own bodies, using things like the heavy-filtering brain/blood barrier. The risk of that sensitive organ getting poisoned, diseased, or biologically manipulated was just too high to risk it.
(Nematodes make what humans call "embodied thinking" look like a joke; the serotonin from their digestive tract is felt by their brains directly.)
People used to die in droves (and at young ages!) of measles, tuberculosis, and a million other things. Even before cities, herd-living put us under immense pressure to develop a pretty intensely specialized immune system. If we had to worry about giving a disease a highway to our central nervous system, that would be... very, very bad. It might even guarantee that such a species would never get to centralize such things at all, such a force is the risk of infection.
Not to even get into the possibility of physical brain-hijackings by concepts (like memes, but oh-so-much-worse!), or even just catching communication-transmitted Kuru... but here's a pretty vivid speculative description of just how bad being an evolved "open book" that granted others write-access would probably get.
And with regards to the "benefits" of open communication -of information conveyed in a language that's very hard to fake- we do still have some information transmitted in body-language and words. That certainly captures some of the benefit. But it bears pointing out that we're a species that un-evolved any obvious presentation of whether a female is in estrus, and has very strong inhibitions around trying not to gain information from each other's body odor. "Complete, total honesty" is not something evolution typically selects for, and it didn't veer entirely that way for us. Even in the less-cutthroat modern era (at least, compared to our distant savannah past), Greg Egan's Closer feels like a pretty realistic depiction of how we might feel about it if we ever did fully share our mental experience with even one another person. I'll avoid spoiling it too badly, but we'd probably quickly uncover a lot of things about one other that we wished we didn't know.
Some Living Approximates
Bacterial mats, giant networks of fungi, and eusocial insects with strong genetic kinship might have strong enough evolutionary incentives for this to line up, although higher relatedness actually exacerbates the infection concern. And between the cells of multicellular creatues, certainly quite a lot does get communicated. Many of these examples do seem to "share their mind" in at least some meaningful sense. They transmit a lot of information and orders to one another, and have a communal decision-making process at varying degrees of centralization/decentralization. Pheromones for insects, various signalling secretions from bacteria, the oodles of transactions and deliveries between our cells at every moment... combined these can be very high-bandwidth. Almost incomprehensibly so, if you've ever seen attempts to measure and chart such things.
Ants could practically be said to have a pheromone "language," complete with clan-identification tags. And as a way to selectively trigger a highly-specific neural pathway, or activate a known set of behaviors in a conspecific with the same brain-configuration, pheromones are not a bad way to go? The behavior patterns pheromones set off can get oddly specific at times.
And... ants also get tricked by pheromones into feeding the brood parasites that eat their own young. And what we call "bacterial sex" (high-bandwidth communication of DNA?) is actually virus-esque plasmids trying to transmit themselves to new bacteria, like an infection. Some plasmids might even come with addiction molecules, which is an extra-douchey way for an plasmid to convey "replicate me, or die." And in coordinated bacteria, you do sometimes see defectors. So... it's still pretty manipulable, and it sure gets manipulated.
The more stereotyped behaviors you can set off through external signals, the more you have a "broader attack surface," in the cybersecurity lingo. And biological parasitism is ubiquitous, and fractaline, and adaptive, and uses any damn attack surface it can get.
Humans? A fluke. Parasites are evolution's true darlings.