Why isn't implementing scientific libraries in software considered equivalent to a publication?
I am not sure what field you are in, but to me, this question almost doesn't make sense.
Nothing is equivalent to a paper ... except a paper. You may as well ask why doing research without publishing it isn't equivalent to a paper. That's just not the model we have.
In my fields, if you publish a library/toolbox, you write a paper on it. If you put up an open dataset, you write a paper on it. If you do research, you write a paper on it. There are many people who focus on developing toolboxes, and many people who might never touch development at all and will only ever publish "actual" science.
In other words, it's not validated and "counts" until it's been at least somewhat peer-reviewed.
You are mistaken in your assumptions. Scientific software is valued and there are now in fact a number of journals dedicated to exactly this -- I know this among other reasons because I happen to be the Editor-in-Chief of the ACM Transactions on Mathematical Software (ACM TOMS). There is (actually: was) also the Computational Science and Discovery journal, and then there is Computing in Science and Engineering (CiSE). All of these publish articles that describe computational software.
More generally, there are substantial efforts at making sure that the authors of scientific software get credit that is comparable to other kinds of publications such as journal papers. I would encourage you to take a look at the WSSSPE series of workshops, the Force11 task force, and a number of other efforts to ensure that software can be cited in much the same way as papers can (e.g., via the Zenodo service).
Software is a research product has value and is seen as valuable, but you as a researcher need to ensure that you extract the value out of it. Extracting value out of any research product requires doing some kind of formal write up and distributing the ideas among the relevant audience. For software, this is many times in the form of a software publication. Many traditional scientific computing journals take a software paper and these papers can do quite well in terms of citations. For example, Shampine's "The MATLAB ODE Suite" is one of the most read and cited articles in SIAM Journal of Scientific Computing.
Taking that paper as a reference, notice that it identifies the problem it's solving, explains how it had to modify the algorithms to achieve its goals, and the kind of results it can achieve. This then gives a citable resource and many of the people who use the paper will then cite this source. Software tends to be under cited, but since it can get so widely used you will find that successful software has some of the most cited publications. Another classic journal like this is ACM Transactions on Mathematical Software (ACM TOMS). More recent software journals are the Journal of Open Research Software and SoftwareX.
In a slightly different direction, the Journal for Open Source Software reduces the importance of the paper by making the software and its documentation part of the peer review. As an example, Optim.jl has a JOSS paper which you can see is much shorter than traditional academic papers, but at least summarizes the value of the software in a citable form.
When applying for jobs, you'll notice that people naturally assign a different value to software papers. While it is definitely changing, some professors I know take hardline stance that software is just implementation and doesn't extend knowledge and will value it less than other papers (which is why these exact points should be refuted in a publication about said software). In contrast, some individuals care more about impact or citations, and so a widely used software will have a very large effect on them. Generally in my experience, the value of software seems to be increasing over time, with senior professors more likely to take the former stance and newer professors more likely to take the latter.
But my advice would be to just do whatever you think has a high impact to the research community. If it does have a big impact, people will notice, and sometimes just being noticed is what you need.