There is a vast difference between a student reading from a textbook and a researcher / scientist reading studies and/or papers.

As a student you are to be directed* in your reading by an expert in the field of study that you are learning from. In many higher level courses a professor will assign multiple textbooks and assign reading from only particular chapters of those textbooks specifically because they have vetted those chapters for accuracy and alignment with their curriculum.

As a researcher and scientist a very large portion of your job is verifying and then integrating the research of others into your domain knowledge. The whole purpose of replicating studies is to look critically at the methodology of another scientist and try as hard as you can to prove them wrong. If you fail to prove them wrong and can produce the same results as them, they have done Good Science.

A textbook is the product of scientists and researchers Doing Science and publishing their results, other scientists and researchers verifying via replication, and then one of those scientists or researchers who is an expert in the field doing their best to compile their knowledge on the domain into a factually accurate and (relatively) easy to understand summary of the collective research performed in a specific domain.

The fact is that people make mistakes, and the job of a professor (who is an expert in a given field) is to identify what errors have made it through the various checks mentioned above and into circulation, often times making subjective judgement calls about what is 'factual enough' for the level of the class they are teaching, and leverage that to build a curriculum that is sound and helps elevate other individuals to the level of knowledge required to contribute to the ongoing scientific journey.

In short, it's not a bad thing if you're learning a subject by yourself for your own purposes and are not contributing to scientific advancement or working as an educator in higher-education.

* You can self-study, but to become an expert while doing so requires extremely keen discernment to be able to root out the common misconceptions that proliferate in any given field. In a blue-collar field this would be akin to picking up 'bad technique' by watching YouTube videos published by another self-taught tradesman; it's not always obvious when it happens.

This is a good point. It is not humanly possible to verify every claim you read from every source.

Ideally, you should independently verify claims that appear to be particularly consequential or particularly questionable on the surface. But at some point you have to rely on heuristics like chain of trust (it was peer reviewed, it was published in a reputable textbook), or you will never make forward progress on anything.

> When I read something in a textbook I blindly believe it, depending on the broader context and the textbook in question. Is that a bad thing?

It is if what you read is factually incorrect, yes.

For example, I have read in a textbook that the tongue has very specific regions for taste. This is patently false.

> Keep in mind that research scientists need to keep abreast of far more developments than any human could possibly study in detail. Also that 50% of people are below average at their job.

So, we should probably just discount half of what we read from research scientists as "bad at their job" and not pay much attention to it? Which half? Why are you defending corruption?