Why programming languages don't provide simple encryption methods?
It's not nearly as simple as you imagine. When you encrypt a string, what character set is it in? What characters are permissible in the output? There are too many unusual combinations of code requirements and security requirements for a "one size fits all" to make sense.

But if you think one size does fit all, sit down and write it. It should take you an hour or so. You'll have it forever, can share it with other people, and this problem will be solved. I think you'll find everyone needs it to be a little different and you'll be right back where we are.

Here's a function to set the character set. Here's a function to switch between hex and base64. Here's a function to add padding. Here's a function to add a checksum. Here's a function to set the key in hex. Here's a function to set the key in base64. And so on, and so on, and so on.

Fundamentally, encryption is not easy. Threat models differ. Requirements differ. One simple "this encrypts" interface is much more likely to be used to disastrous results than good ones.

There's also the attractive nuisance issue -- things that are neither easy nor safe probably should not look easy and safe.