Almost everything about cameras, lenses, sensor and film sizes are specified in metric units. So why are common tripod mounts (e.g., 1/4"-20) not metric?
I'm just curious if there's an interesting bit of history to explain this.
First of all, tripods are things you really want to be compatible across cameras & between tripods: no-one cares if the screws which hold your Leica together are compatible with the screws that hold your Nikon together, because you're not about to screw bits from your Leica onto your Nikon (actually: camera repair people kind of do care as it means they have to keep stocks of fewer different sorts of things). But no-one wants to have to buy a new tripod each time they buy a new camera, other than camera makers, and very few camera makers have ever been dominant enough to drive that kind of awfulness onto people. (It's perhaps significant that in many cases they have been able to do this with lens mounts and still do to a great extent, although even here consortia have appeared (μ4/3 for instance). But lenses are things that camera makers can argue really make a difference, while a tripod is, well it turns out they matter too but they're a lot harder to sell as being somehow special). And not all lens mounts really can be compatible: SLR mounts & can't really have flange distances as short as mounts for cameras without mirrors, and if you try and use SLR mounts for mirrorless cameras you get weird things like the recent Sigmas.)
Secondly tripods are expensive & last a long time: unless you damage it you can use a tripod for many, many years.
The result of this is that, once a tripod screw thread standard exists, there's a really strong incentive for it to persist for a very very long time.
Tripod mounts in fact use a standard called Whitworth which was standardised in 1841, in Britain. For the purposes of mounting a tripod it's close enough to UNC which was standardised over a century later (the thread angle is slightly different, but the diameter & threads/inch are the same).
And this, I think, answers the question. In the early days of photography in the late 19th century if you wanted to sell your tripod the biggest market in the world, by a large margin, was the British empire (in 1870 the British empire was about 25% of world GDP, while the US was about 9: the US caught up sometime during the 1914-18 war), and they used Whitworth (and a number of other standards, but Whitworth was pretty dominant for most purposes). And most tripods would have also been made in the British empire. So if you were outside the British empire, then you made your cameras and tripods with Whitworth threads, and this has stuck ever since.
[Apologies if this reads like some kind of misty-eyed wasn't-the-empire-great romanticism: it's not meant that way.]
[Consider this re-framing: What would be gained by having a metric tripod mount?]
The core of the reason why metric tripod mounts are uncommon is a combination of:
The early part of the 1900's saw a wide range of tooling standards established early on before metric tooling became popular/common. This established a long lasting snowball effect in some industries like camera gear.
Once a large portion of gear on the market was already using the 3/8th and 1/4-20 standard, it became far less practical to introduce new gear that used metric threading.
Little to no new near that used metric threading meant that there was little to no economic reason to make a change.
Screws for the camera assembly had a far lower barrier to entry into metric - It doesn't matter what standard you settle on, or even if every screw was machined to a different standard in the camera body. If no one is expected to replace the screw themselves or have it screw into something else outside the camera, then it really didn't matter what you used.