Poor eyesight does not result in more eyes to compensate that flaw. Remember: evolution is mostly random mutations.

At some point, a mutation resulted in multiple eyes - some individuals had really good eyesight, others really shitty eyesight and everything else inbetween. The reason why spiders don't have extremely great eyesight is because those individuals - while passing on their genetic material - did not have a higher chance of survival compared to individuals with worse eyesight.

Why? Because eyes are not everything. There are other parts of a spider's body that help gather information and other random mutations affecting those body parts/functions, respectively "skills" contributed to the overall chance of survival.

A spider was able to pass on its genetic material because in its lifespan, the sum of all mutations provided a more or less successful "survival package" that helped it overcome all situations until it was able to mate. And obviously, luck is involved, which means that flaws (unfavorable mutations) were be passed on as well.

So over millions of years, most spiders did not develop better eyesight because it simply did not provide a massive advantage to survival, thus those individuals with mediocre eyesight had the same chance to procreate, further spreading the "mediocre eyesight" gene.

There probably were niche habitats in which better eyesight was a huge advantage, thus those individuals probably dominated the genetic pool and over time eliminated "bad/mediocre eyesight" genes - but possibly also ended up passing on mutations that weren't that great - but overall still not bad enough to lead to extinction.

In other habitats, eyesight wasn't really that important, so there one would find more individuals with "bad/mediocre eyesight" gene, while other mutations turned out to be much more relevant, thus they passed on genetic material that was essential for survival in that particular niche.

The complete set of advantages and disadvantages has to be taken into account when looking at how a species evolved and why it evolved in that particular way. During the process of evolution, it is not the strongest and best individuals that continue to spread their genetic material, but those who have the most beneficial "package" that helps them adapt more successfully to certain conditions.

At the end, the number of eyes a spider has these days is just the result of a specific mutation turning out to be beneficial in combination with all other mutations. This also includes biochemistry, metabolism, etc. At some point there probably was a spider with 12 eyes or more due to a mutation - but even though more eyes might have provided a certain advantage, it probably also increased the energy consumption, thus such a spider had to hunt more often or hunt bigger prey, which also increased the chance of failure. At the same time, just two eyes would have been not too bad either, but that might have had negative impact on defensive strategies as well as hunting, thus, that mutation did not last long either.

So if we take a look at what a species looks like today, we have to take everything into consideration. There is always a reason why certain genetic material is the way it is now - while random mutations provide different "parameters to operate with" (for the individual), that "starting gear" isn't only limited from a "trait" point of view, but also in its application.

As you can see, the complexity is massive. What we observe today is the outcome of a lot trial and error from a genetic point of view. Each individual is thrown into the wilderness with a particular "set of tools" (genetics) and those who use their tools efficient enough and/or are just lucky enough will be able to pass on their genetic material. Even if your set of tools is the best, but you fail to use that efficiently in a specific niche or just have bad luck, you will not be able to pass on your amazing genetics.