Let's take a simple claim:

On Earth's surface, acceleration due to gravity is ~9.8m/s^2

I haven't tested this, but here you are reading it.

Did the person who I learned this from test it? I suspect not.

Did the person who they learned it from test it? I suspect not.

Did the person who they learned it from test it? I suspect not.

Did the person who they learned it from test it? I suspect not.

Did the person who they learned it from test it? I suspect not.

...

Did the person who they learned it from test it? I suspect not.

Could anyone test it? Sure! But we don't because we don't have the time to test everything we want to know.