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It’s more that they are easy to breed and to have a lot of them. The accuracy of the tests is very low. Even tests on animals like monkeys, who are much more similar to humans, are not very accurate.
This is a big argument of the people, who are against animal testing. Testing on nonhuman animals will never be as accurate as on humans. However, as you can imagine, it is hard to find human volunteers.
It’s not exactly a “biological link” rather rodents are an animal that breeds quickly and is easy/cheap to feed. So they make excellent initial test subjects. As mammals they share most of the organs that humans have so it can be a decent way to test things.
Using primates like chimps would provide us with a closer comparison, but chimps are an expensive animal to keep. Especially if the thing you’re testing may or may not kill the subject. All that is ignoring the simple revulsion people have to animal testing on primates moreso than rodents.
Rats are small and reproduce relatively quickly. If you want to test something on 100 rats you need a lab room, if you want to test it on 100 pigs (a much better analog to humans) you need an entire large building. You can also pick a rat up and inject something into it, but injecting a pig is a whole ordeal.
Rats are easy, and due to how often they are used we have a good understanding of how they react to various things which can save duplicating effort in some cases.
We are mammals , just like rats, and by and large have very similar physiology and anatomy. Rats however are fast breeders, easy to raise, mostly diurnal, not dangerous to handle etc so that makes them good lab candidats.
In addition to what others have said, primates and rodents are also the two lineages that are closer related to each other than the other famous mammal lineages like carnivora, ungulates, etc. are to primates. Both are [Euarchontoglires](https://en.wikipedia.org/wiki/Euarchontoglires).
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