Why make wakeup imprecise by default? The slop allows for some coalescing of nearby (in time) events into a single wakeup, saving power[0]. This article provides some figures that give a good intuition for why this might work: https://arstechnica.com/gadgets/2013/06/how-os-x-mavericks-w...
See also: tickless kernels in general[1]; FreeBSD switched to a tickless kernel with coalescing about 5.5 years ago[2].
I agree that this can be beneficial, but do any BSDs support any laptop hardware as easily or as well as Linux does? Even assuming that hardware support is there, does the battery life even compare well to Linux on the same hardware, let alone Windows 10?
If one of the best kept secrets of open source is that some BSD somewhere has great hardware support and battery life better than Windows and Linux... that would be awesome!
To answer the broad question, yes, some BSDs (FreeBSD) support some laptop hardware[0] as well as Linux does. And yes, the battery life is comparable. FreeBSD has good support for idling in C-states, which is where most of your CPU power saving comes from; support for setting and managing P-states (which no longer have much impact on modern Intel CPUs, but mattered historically) via e.g., "powerd;" and support for wifi power saving modes and screen dimming (other big power draws).
The question that was sort of implied but not directly asked might be: if you pick a random laptop, do I think it's likely a BSD will support it better than Linux? And there I think the honest answer is "no." Linux definitely has an advantage in breadth of driver coverage.
In many ways FreeBSD is a well-kept "secret," but it's not a panacea.