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Even C++ is 38 years old.
And these are just the languages; the systems our current OSs are inspired from were designed without much security in mind: understandably so, considering Unix was also created over half a century ago, long before the Internet. GNU was created nearly 40 years ago, and even Linux released over 30 years ago, only 2 years after the world wide web was invented, so cyber attacks were not very common.
At least with iOS and Android we've been able to have a better permission model than most desktop OSs (e.g. applications have to ask permission to use the camera), as well as better sandboxing (one application can't just read files from other applications: users explicitly have to share things from one app to another, and can revoke permissions at any time.)
But I agree with you that they're ultimately built with primitive languages and questionable design decisions. That's not a criticism of anyone who designed those systems and languages: the fact that they're widely used half a century later shows how incredibly good they were for the time! But a lot has changed in 50+ years (and especially the last 20-30 years with the Internet), and we've learned a lot.
We do get new languages, though I feel like we still have much to improve: the fact that Electron is so widely used for desktop applications (even ones that aren't web apps) shows how far we have to go for languages and their frameworks. But at least there are people trying to make better programming languages and frameworks.
Unfortunately the outlook on OSs seems grimmer. There are some hobbyist OSs that are extremely impressive, but I'm not sure any of them look likely to replace the OSs we use for servers, desktops, phones or other devices. With all we know today, we could design and build an OS that is far more secure than what we have today. But the costs of doing so would be enormous, so I understand why no organization has taken on this task.