Because your body can (and sometimes does) overshoot the mark and kill things it isn't intending to kill with heat. It also uses up your stored energy meaning you must eat more and more often.

Better to make your body stop overheating itself, and trap heat with coverings. If you get too hot, it can be modulated by reducing or adding cover. And external heat like a water bottle can bring temps up quickly, and keep you at raised temps safely.

There is also the discomfort from a fever as well. Too much discomfort will lead people to make possibly bad decisions. Pain management is always something to keep in mind.

As soon as I start to feel like I might be getting sick, I immediately take as hot a bath as I can stand for about an hour then immediately cool off with cold water. Seems to do the trick quite well the majority of the time. My partner will go through the full illness while I can mostly nip it in the bud.

You don't usually need to lower the fever. We do it for comfort, mostly. Being feverish feels bad. One of my kids can tolerate a fever so I don't usually give her fever reducing medication. The other one gets so miserable that she won't stop screaming, so I give her the medication.

We don't like to be hot

It's not necessarily a good idea to do that though, particularly if you are able to endure the fever

Because GET BACK TO WORK

Paraphrasing my doctor: infections have evolved to the point the fever isn’t effectively contributing to fighting the infection, but we never evolved to stop making the fever. Since it doesn’t do us any good, we might as well be as comfortable as possible while our immune system does it’s part.

Indeed you should if you are not too hot. For diseases that can be cured otherwise do that first (antibiotics for batterial infections), there are still not good medicines for virus infection, so better let your body fight the infection and decrease your temperature only if too high

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Because it doesn't feel nice.
But if you're lowering every fever, you're doing it wrong.

Paracetamol also isn't particularly good at reducing fever, even though it is often used with that intent. Ibuprofen is generally more effective