why does my powersaw keep tripping the breaker?

Interesting. Assuming you are using a dedicated circuit (no other items in use on it) then I would have to say:

a) the saw is old and the armature is dirty

b) the breaker is worn

If you can, replace the brushes and clean the armature.

Try another breaker in the panel if one is available.

Look at the inside of the panel cover for a brand name. Compare that to the brand of breaker, they may not match. Get a replacement of the correct brand and type.

The inrush amperage shouldn't be near 20 amps, 12 to 15, but not more than 20. The breaker should be able to handle an inrush of about 25 amps for a second or two. (if "dtstellwagen" see this he may take the time to give the technical and mathematical specs involved here. Me. I'm a "seat of the pants" kind of guy.)

That is a good question on why it trips when you go directly to a socket but not with an extension cord. Usually it would be the opposite condition. That being stated as correct and it is the same outlet, I would suspect a bad cord cap on the saw as the most likely problem spot. After that I would be certain that all of the dust and shavings are removed from the saw.

You might have a short in the saw. Observe the sparking through the vents for signs of irregularity.

Second, you might have too many other devices on the circuit or the breaker is bad. Breadkers do become defective.

Get saw PAT tested for electrical leakage issues between motor wiring and motor/gearbox frame. Could also be failing suppressor capacitor.