- it is more secure than plaintext that you had before (even if only by a slight margin, as you're not experienced in crypto).
- you're an interesting target yourself (for example: top politician, financial access / banksbanks, celebrity status, have access to rare information, etc)
- you're perhaps not interesting yourself, but there is a lot of "you" using the same security method. In other words: popularity. Sure, facebookFacebook does not contain much of top secret-secret data, but having a way to access ANYONEs facebookanyone's Facebook account is going to be a very good incentive for hackers to try to crack its protections protection.
- bad luck (eg. some hacker is bored and just happen to take a pick on your site, transforming it to point 1. above)
Now, sure TLS1.3+AES256-GCM/SHA512 is great thing today. Nobody can spy on you (except NSA and friends, but if you made an enemy of them, you're done already). But there is an enormous amount of people using it, and thus it becomes enormously profitable to crack it. So it will happen.
However, your toy-crypto chat, used in the whole world only by you and your friend to talk about slightly embarrassing fetishes, will likely remain hidden from everyone but legitimate parties for all eternity. Simply because nobody cared enough to give it a minimum of effort needed to crack it, and it wasn't popular enough that is was decrypted by default by some PRISM-like mass surveillance.
But I'm wondering if, in the long run, we wouldn't all be much better if we had billions of obscure and insecure ciphers each used only by few people; as apposedopposed to few supposedly uncrackable ones that everybody uses (but which are secretly flawed - which is known only by 1984-ish govermentsgovernments and other murky elements.
As (anecdotal) "evidence", every month I help fix dozens of broken wordpressWordPress/joomlaJoomla/etc sites which were "sooo secure" just a few months/years ago. But strangely enough, every now and then I stumble upon properly written (eg. verifies input :) old perlPerl or even shell scripts, using insecure RC2 or even just hackish XOR, which still standstands strong after decades. Just because there were different (and not popular) and nobody cared enough to spend time to crack them.
As your biggest problem nowdaysnowadays is probably not that you will get targeted, but instead that some automated bot will exploit some hole in a popular piece of software which you didn't patch quick enough. As there is a limited amount of crypto-crackers, and there are most of the time chasing bigger cat than you. So you'd most likely become low-priority ticket and thus fade into oblivion.