1) It's a mechanical device under huge manual and braking stress. They break. They can't not break. Maybe you haven't seen it, maybe you just don't use the break or own enough cars or care enough to check.
2) I used to buy old cars. 5-10 years old. Until recently, I never bought new. I used to change car when they didn't pass the relevant tests, so I bought a lot of second-hand cars. Almost ALL of them had parking brake problems. From "it doesn't do anything" to "it needs a serious amount of adjusting" to it literally could not be released once activated.
3) My dad does all my repairs/maintenance as he worked in the trade for decades. Ask him if mechanical parking brakes never fail.
4) My new car, a year or so old now, has electric parking brakes. I distrusted them, like you. When my car was new, I took it to a couple of off-road locations to test lots of things (I'm not a boyracer, speed was NOT one of them, I'm literally talking about "Oh, I don't like that... how does that work if..." scenarios) - best way to get to know and trust a car is to actually activate these things in a safe place.
Electronic parking brake? Massively outperforms a mechanical one. I could not make it not activate on demand. I could not make it activate inadvertently (it appears the button/toggle that controls it has debouncing that's undergone a lot of testing to avoid inadvertent activation, but yet work whenever you need it to). And there's a reason I couldn't make it work inadvertently... the parking brake is not just a parking brake but your only non-hydraulic method of stopping the car in an emergency.
I deliberately read the latest car design requirements from the government and, at least in my country, the parking brake must still operate independently so it can be used in the car of a brake failure. I was worried they were obsoleting a safety backup, but in fact the requirements are much more stringent now than most of the old cars I used to drive.
And so I took it on a non-public road. And I poodled along and pressed the parking brake. Holy shit did it stop. Even on gravel. Okay, so I got braver and braver and asked it to stop me from faster and faster speeds (never going stupid, but still - on a motorway this might need me to stop the car before it hits a line of traffic, late, after I realise the normal brakes don't work).
HOLY SHIT. You have no idea how effective it is compared to a traditional cabled parking brake (no handbrake turns, for sure, because you just don't get time, but then I would never attempt that anyway). I only avoided whacking my head on the steering wheel each time because I knew it was coming after building up from the slower speeds (presumably airbags would kick in in a collision, I'm not testing that though!).
I tested the "auto-release when you drive". I couldn't make it release when I didn't want it to. Literally, you have to have enough driver-instructed forward motion that you would hear the brakes screech anyway if you did move and it deactivates a fraction of a second AFTER you're actually moving against the brake.
Hill starts? I actually worry now that with electric parking brakes, you never have to be able to do one properly. They make it that reliable and easy. I've literally never rolled back, not even an inch, unless you are absolutely 100% negligent and wait for the brake to release and then suddenly let get of everything and it still takes a second or two to roll back because the car has to have been moving forward to release the brake.
And if I hold the foot-brake for a few seconds while stationary, it knows that and just puts on the parking brake too. It doesn't do it while moving at even a tiny speed where you can barely see the wheels moving even if you're doing that by holding the normal brake. It knows the difference.
Someone put a lot of testing, thought and effort into my car's electric parking brake. I couldn't make it do things I didn't want, even when I was completely abusing the mechanisms to try to fool it.
And, reading online, it warns you if it fails in any way, which mechanical ones do not. The number of people whose cars I've driven only to be told "Oh, no, you can't use the handbrake, it doesn't work" and there's NO indication of that. The electric brake? The first single failure and you get the brake warning light which you can't get rid of until you've OBD'd it away. And it reappears the second it goes wrong again.
5) Mechanical light switches in my house have been mostly replaced. Because although they might "work" they have severe limitations (e.g. the number of switches that can be joined to control a single set of lights, without electronics, relays, etc.)
There's a reason that people are moving away from mechanical, moving parts towards electronics that can verify themselves. Nobody is saying "they can never fail", we're saying "they fail much less and most of the time you'll know they are failing", which I've found to be a big problem with mechanical parts with older cars.
Electronic ignition (i.e. digitally controlled, not just a mechanical replacement).
Electronic parking brake.
Electronic braking systems.
Electronic power steering.
Electronic injection and timing.
If you don't like it now, I suggest you stop driving, because there's no indication that mechanical means will ever return or that they are even capable of meeting modern safety standards any more. And when those old cars die, you will never see another built like that.
Yeah, the complexity in something that used to just be four cylinders and a few belts is scary. But there's a reason. They do far more than mechanical ever could, far better.
Nobody claims they're fail-proof. But given the old wrecks I used to drive, I have far more confidence in an self-checking electronic part that's almost impossible to fool, over whatever some bastard has done to the parking brake cable over the years.