You know lab mice aren't just grabbed out of the sewers, right?

Humans only discovered hygiene somewhere in the last couple of thousand of years. Evolutionary pressure for large animals works on time lines of hundreds of thousands to millions of years. Before we got cleaner (and also after that) we also lived in unclean conditions, often are still covered in fleas and lice and we are still one of the greatest spreaders of disease. Humans and mice are extremely similar in many ways, just because we have a large brain doesn't mean we are somehow no longer the animals we always were. We share much of our evolution with mice, our cells are extremely similar and we share 92% of our DNA.

Mice are an excellent point of comparison to humans. And because they are small, live short lives and grow fast, they are excellent to serve as a basis for testing. However it's also worth remembering the mice aren't the starting point, nor are they the end point. It's just one of the steps in between and many other species and techniques are used. In a lot of cases, mice aren't used at all, but some other test is done.

It's also like people seem to think that researchers are just doing random crap to mice and seeing what works. Like I said there is a lot of stuff that comes before and a lot of stuff that comes after. Tests with mice are often done to research something very specific, with a carefully considered method of testing and expected outcome. If someone thinks of something so hyper specific to humans, they would simply not do any trials on mice since that wouldn't yield any results. These days we've also gotten extremely good at growing cells and complex clumps of cells at large scales for not much money. And these can be actual human cells with actual human DNA and biological processes. This has made animal testing far less necessary than it was in the past.

Sure at some point if something is very promising but there are doubts about some complex interaction that might be an issue, animal testing can be useful. But if the thing to test is something so specific to humans, an animal closer to humans would be used, for example pigs or some monkeys or apes. And if those doubts aren't there it isn't like animal testing is a required step, it is possible to go to human trials without it.

Of course this depends heavily on what it is you are trying to do. For drugs for example animal testing is often done, but often not to figure out if it works or not. But to figure out what sort of dose is needed for enough to be absorbed, but not so much the drug is wasted or the patient would experience a lot of side effects. It's pretty easy to do a short trial on some pigs and have the first human trial get the dose right straight away. At this point it's more of a regular way of doing things than something absolutely required. In a lot of places regulation will require some animal testing, especially for drugs, but these days with better lab tests and simulations it isn't strictly required.

So it might be a fun shower thought, but it isn't really how stuff works in real life.

It's just an early testing phase, more of an "ok, this drug kind of did what we want in mice, and the mice didn't explode. It's time to move to the next stage of testing on something more human analogue, like a pig."