I'm not saying planned obsolescence isn't a thing (because it is), but that's not the only reason. Making phones smaller, lighter, faster, and more feature-dense all mean that the phone has to be built with tighter manufacturing and operating tolerances. Faster chips are more prone to heat and vibration damage. Higher power requirements means the battery has a larger charge/discharge cycle. And unfortunately, tighter operating tolerances mean that they can fall out of those tolerances much more easily.

They get dropped, shaken, exposed to large environmental temperature swings, charged in wonky ways, exposed to hand oils and other kinds of dirt, and a slew of other evils. Older phones that didn't have such tight tolerances could handle all that better. Old Nokia phones weren't built to be indestructible, they are just such simple phones that there isn't much to break; but there's a reason people don't use them much anymore. You can still get simple feature phones, but the fact remains that they don't sell well, so not many are made, and the ones that are made don't have a lot of time and money invested in them.

Now Voyager is an extremely simple computer, made with technology that has huge tolerances, in an environment that is mostly consistent and known ahead of time so the design can deliberately account for it, had lots of testing, didn't have to take mass production into its design consideration, didn't have to make cost trade-offs, and has a dedicated engineering team to keep it going. It is still impressive that it has lasted this long, but that is more a testament to the incredible work that was and is being put into it than to the technology behind it.

There’s also the fact that the mass market wants their cheap shit. Make something to last 10x as long at 3x the price, and sure folks will buy it but the market share would be minuscule.