A lot of things aren't standardized because there are so many ways to do the same thing that could be right in different circumstances.
Take your innocuous example of validating multiple textboxes. We all do it, should be straightforward, right?
Well let me point you to http://st-www.cs.illinois.edu/users/smarch/st-docs/mvc.html which is a classic paper describing how to solve that exact kind of problem in Smalltalk using the MVC paradigm. It handles a hard generalization of your problem, but assumes that you're working in a standalone application. It would take serious thought to apply it to a web environment. (That particular solution is the original root of Microsoft's Model-View-Presenter paradigm, which is used to solve a very different problem from validation.)
OK, we want to work in a web environment. There are a lot of technology stacks in a web environment. Like it or not, validation is closely tied to how we organize the generation of web pages. Which means that if I'm using a different technology stack than you (I am actually, I don't use any Microsoft technologies at all), then a design pattern that works for me is likely to be different from one that will work for you.
Now suppose that you have two text fields that are supposed to be a range. So the valid in the one has to be less than or equal than the value in the other. My little validation mechanism looks at just one text box at a time. It has no way to look at two at once and compare them. You start to dive in and try to fix that, and realize that you'll have to rewrite a big chunk of it. What I wrote wasn't bad. But it doesn't work for your use case. Guess what? You'll need to take a very different approach than what I did. And when you've solved that problem, if you show it to me, it will be bad for me because it forces more complexity on what is, for me, a trivial operation.
Luckily it isn't a lost cause. Every codebase develops its own standards for how to handle lots of different kinds of problems. Many of those standards are shared with a wider community. Many more are private to that organization and that codebase.
But now suppose that you work for a company who has solved this problem. Then you're on a site like this, and someone says, "I need to validate a bunch of textboxes, how do I do it?" You know how to do it, you do it every day. But for legal reasons you can't just cut and paste your validation stuff. And your validation stuff is tied to the object-relational mapper that you're using, so even if you were willing to share, they can't easily use it without pulling in a lot more of your infrastructure. And the result is that, even though you are sitting there with a working answer to the questioner's problem, you can't just tell them, "Do this."
(Note that if you're using popular open source code, the odds of being able to share go up, and so do the odds that the other person already has the right dependencies go up. This is one of the appeals of open source.)