The short answer: because platforms that can interface with AirTags aren’t ubiquitous (yet).
Does the Mac chip M1 contain UWB – the communications medium for AirTags? Almost certainly. OTOH, I can’t find a single news story mentioning UWB on the chip. Apple’s page describing the chip doesn’t mention it. Did any of the post-presentation briefees ask about UWB on the new Macs? Will iFixit be able to identify a U1-like UWB section of the M1 chip in their teardowns? Hopefully, we will get more information in the next few days.
I think that Apple is designing UWB for far more than “find my [XXX]”. Applications using AirTags for some sort of VR/AR work/gaming would be a lot more useful when the technology is available everywhere. Motion-tracking games using the precision of UWB look like a slam-dunk. Apple has a much better story for “every device” motion-tracking apps when every platform has UWB. The leading edge of Mac users will have their M1 machines in the next 2-3 months; I think early next year is the time for AirTags will finally be announced.