Where do all the missing cats go?

Im guessing a large number are hit by cars but then either the driver would take it to the vet or someone would see the cat on the road.

Not necessarily. I hit a cat several years ago on the outskirts of my village, travelling ~30mph in darkness. It darted out of the hedge and straight under the wheels, no chance of stopping or avoiding it. I emergency stopped, turned round, stuck the headlights on full beam and there was nothing there. No sign of anything at all. I went home, got a torch and DH, we both searched for ages, and couldn't find it. I assume the cat was alive and mobile enough to have gone back into the hedge/ditch, and then there was no chance of us finding it. We went back in daylight and still couldn't find any sign of it. I expect it died overnight, but without a clear view of it I couldn't tell you what colour it was, never mind identify it well enough to reply to a missing cat poster.

Road accidents- it can be very hard to identify a cat corpse, the bodies are fed on as carrion or disposed of by street cleaners.

'Adoption'- our cat was 'adopted' by a woman who started feeding her and then decided she should be 'indoors' so she couldn't come home anymore. After a few months cat got mildly ill, and the woman decided she didn't fancy paying the vets bill so traced the chip and returned her to us.
Similarly, someone I know 'adopted' a neighbour's cat because it was 'too thin'. Again, same tactic, started with feeding and then stopped it leaving for it's home by making it indoors only. Eventually, neighbours distraught daughter reclaimed it.
People seem to have very skewed views of what a healthy weight is for a cat. Facebook posts on the community forum often feature cats who are perfectly normal or even a bit podgy being described as 'clearly malnourished'.

Some might be taken for lamping (although it's much easier to get hold of unwanted kittens than it is puppies for this purpose). Some do genuinely wander off and can't find their way back- lifespans for feral cats are quite low I think.

PILs ‘stole’ cats over the years, just by feeding them and the cats chose to stay with them, they were free roaming.

The local council should have a phone number you can ring to report a dead animal, and someone will pick it up. They usually keep a record of cats and dogs, so if your pet is missing that's one place you should ask if they've seen it.
Locally, people were encouraged to shave a strip down their cats back as there was such a problem with them being taken for the fur trade, that seems to have stopped as its now a specific criminal offence to steal a pet (Pet Abduction Act 2024).
https://www.legislation.gov.uk/ukpga/2024/16/contents

Nobody is obliged to take a cat to the vets if their car hits them - unfortunately. I wish this would change.

It took us 3 days to find our cat who'd been hit by a car and we had searched for him. I think the postman said he'd seen a body. Whoever hit him obviously didn't stop.

We have occasionally lost other cats for a couple of days when I think they have got locked in somewhere

There was a dreadful time many years ago when some were being taken for the fur trade but I think that has stopped

I don't know the answer. We are very particular about getting ours in by a certain time at night and trying to keep them in in the morning to avoid the morning rush, it also gives the wildlife a sporting chance😀

People steal cats all the time but don't class it as stealing

You'll find there's a good portion of people who's cats "found them", who have a cat that they didn't buy, adopt etc themselves but randomly turned up. People start feeding them etc, and sometimes make surface efforts to find their original home but keep them

Road fatalities, but also just general cat injuries are high. They live a fairly high risk lifestyle of road running, under car sleeping, scrapping, roof jumping and exploring dangerous areas. Other animals will eat cats (if dead) etc, and ill cats will often leave home, or hide. There's a good chance that lots of them will have died in quiet places, bushes, abandoned ground etc.

I remember there being a petition because roadsweepers, and railway workers were just disposing of deceased cats without checking chips etc

A high ranking Dr I knew used to load up his car with neighbouring dcats and drop them off in woods far away.
To keep them out of his garden.
Utter twat.

A lot of them will just be wandering, it happens a lot in the summer, then the owner won't bother to take the poster down when they return. Also probably more than you think are adopted by 'kindly' idiots who fail to check them for chips at the vet.