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Especially in the Republic era when the use of the lightsaber was more common, why didn't anybody use chemical propellants (real life modern guns)? A modern-day assault rifle has at least 10 times the rate of fire and the bullets travel at least 10 times faster than blaster bolts. You could fill a Jedi with 30 bullets in an eye-blink without worrying that the Jedi blocks and reflects all your shots back at you.

Even if the sophisticated automatic firearms never got developed, 10 arquebusiers from the 16th century would have stood a better chance against a Jedi with a lightsaber than 10 troopers equipped with blasters.

Maybe blasters cause bigger wounds, and maybe they are better against armor, but they should not have made gunpowder completely obsolete. Is there any in-universe explanation why nobody uses gunpowder or similar chemical propellants?

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   Gunpowder (or at least projectile) weapons WERE known in SW Galaxy - they are mentioned in canon (including use against Jedi, e.g. by Aurra Sing and some other mercenaries)

   
   
   
   
   

   • https://starwars.fandom.com/wiki/Black-powder_pistol

   
   

   • https://starwars.fandom.com/wiki/Sevari_Flashpistol

   
   
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     Most importantly, there were Slugthrowers (though whether they used powder is not known from current canon).

     
     
     

     
     

     Qymaen jai Sheelal, who would become the infamous Jedi hunter, General Grievous, used a slugthrower rifle during the Huk War.

     
     

     
     
     

     Another (though not directly attributed to specific canon source) quotes from the Wikia:

     
     
     

     
     

     For example, during the days of the Galactic Republic, mercenaries fearing an intervention by Jedi Knights used rapid-fire slugthrowers that were impossible to completely deflect, unlike blaster bolts.

     
     
     

     Aurra Sing famously carried a slugthrower along with her blaster pistols and lightsaber.

     
     

     
     
   
   
   
   


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   As to the reason why it wasn't widespread:

   
   
   
   
   

   • Most people did NOT set out specifically to fight Jedi, and gunpowder weapons are useless in modern blaster combat.

   
   

   • Those that did set out to fight Jedi, often used more appropriate tools (cortosis armor, Yslmmmri, a Dark Jedi of their own, sheer numbers) and didn't need to go through the trouble of firearms logistics.

   
   
   
   

Especially in the Republic era when the use of the lightsaber was more common, why didn't anybody use chemical propellants (real life modern guns)?

They did. It happened on planets with extreme jungle terrain or other conditions which made blasters unreliable. One example is the planet Haruun Kal from the novel Shatterpoint.

It was also a thing in poorer areas. A visual dictionary for I - VI mentions that the posse of moisture farmers who attempted to rescue Shmi Skywalker were armed with both blasters and slugthrowers.

You could fill a Jedi with 30 bullets in an eye-blink without worrying that the Jedi blocks and reflects all your shots back at you.

Prior to Order 66 (perhaps not the highest number of Jedi, but still a high number,) the Jedi Order had something around 10,000 members. The galaxy is too big for those numbers to mean much when deciding the standard weaponry of a soldier. Also, as DVK noted, if you knew you were going to fight Jedi, you would bring something like lightsaber-resistant armor, force-nullifying creatures, or a bunch of cannon fodder. Slugthrowers just aren't the best option, even against Jedi.

Maybe blasters cause bigger wounds, and maybe they are better against armor, but they should not have made gunpowder completely obsolete. Is there any in-universe explanation why nobody uses gunpowder or similar chemical propellants?

From wookieepedia: "Blasters rapidly displaced the slugthrower as the primary infantry weapon owing to their greater ammunition capacity, power, and lighter ammunition compared to slugs." So yes, blasters cause bigger wounds. They also have better logistics and less recoil.

As far as armor goes, standard body armor was extremely resistant to slugs. A visual dictionary states that the white plate sections of storm trooper armor were almost immune to slugs and shrapnel. Only the black sections (weaker for flexibility) were vulnerable. The Ewoks' ability to shoot through that armor to lethal effect is explained - they used incredibly strong compound bows with poisoned arrows (so they didn't actually have to kill with mechanical force, plus it's not a slugthrower anyway.)

Let's not forget battle droids. Slugthrowers aren't nearly as effective against droids as blasters, and most Star Wars movies show at least one combat droid.