There are a bunch of reasons why teaching to the test doesn't work.

1. Because tests are a crappy way of assessing knowledge.

There are students who are amazing test takers, but don't really understand the material. There are students who are terrible test takers, like they have test anxiety, but have an incredible understanding of the material if you talk to them and they work through a problem in front of you.

2. Because it's a terrible teaching methodology.

No one wants to learn about something because it's on the test. That's horrible motivation. They want to learn about something because it explains something cool they could never understand, because it provides a new perspective, because they get to do an exciting thing, because it's a fun competition with others, etc.

When you have to teach to a test, people teach to a test. There's pressure from administration to do it because the merit-based pay isn't just for you personally, it's also for the school as a whole. When test scores don't go up your school gets punished too. So now you drill the specific problems on the test over and over again. Do test scores go up? Sure, by that 0.1 standard deviations we talked about. Does joy go up? Does understanding go up? No.

3. Because tests can only test so much.

Practically, only so many topics can be on the test. There are big topics that are important to know in every class. There's tension here: if you design a test that's in a sense fair for a machine, you pick a random page, a random paragraph and ask a specific question about that paragraph, well, ok, you have a test that tests everything. Sort of, at least at the level of memorization. But, immediately people would say this is a terrible test for a human: why does it matter that my child remember the minutia in page 32, paragraph 3, when there are 7 big topics in this course, the topics that are important to build on for next year, and my child mastered them all? And that's fair criticism.

So now, tests become about the big things. Which makes sense, that's what you need for the future. But that interacts with 1 and 2. So now you drill the big topics over and over again. It becomes a game about memorizations.

4. Because we start teaching test taking skills instead of material

Many people are not good test takers. And that's fine! The goal of tests is not to test if you're a good test taker. It's to test if you know the material. We specifically design tests to avoid testing how good you are at taking tests.

Well, when the stakes are so high at Mr/Ms's Smith's retirement fund is on the line, and St. Margaret's operating funds for next year are on the line, people teach test taking. This is miserable for students. You basically teach it by taking a lot of tests over and over. And then of course teaching test taking strategies.

5. Because it makes losers and winners.

If a teacher and school knows that Jimmy isn't going to make it to grade level, will they work with Jimmy so he can do his best. Maybe catch up a little this year? Maybe find an alternative teaching style. Maybe there's a 10% chance that it will work out for Jimmy and he'll go on to university and do amazing things. No. Teaching to the test and merit based pay means that teachers will dump Jimmy. Even if they don't want to do that, the administration makes them. Jimmy is a liability, sure, but it's worse. All the time spent on Jimmy becomes a liability too. Better to just discard him to the scrap heap, he's unlikely to pass the test anyway. We'll double down on our efforts to help Bob instead. He's middling, he has a 70% chance to pass the test. If we double down maybe we increase that to 90%. That's much better for us. This is terrible for students and it feels really bad as a teacher too.

There's much more that is wrong, this is just a short summary.

It doesn't teach people to become educated, curious, smart, interesting, kind, well-rounded. To ask interesting questions. To want to learn. It forces teachers to turn people into widgets and to discard them like widgets.