Personally I think life originated on earth. We shouldn't look at contemporary prokaryotes to determine how complex prokaryotes were at the start of life. They had billions of years to develop as well and if the more complex ones are fitter than the less complex ones, they will win. E.g. you think rotting plants is a normal thing but there was a time when plants wouldn't and just form giant heaps, forming the coal reservoirs of today.

Anyways, let's assume life originated at some other planet and it has started life at half of the galaxy at the time it reached earth (so we weren't lucky to be "close" to the origin).

Earth is 4500 million years old, and life is assumed to have existed here since 4400 million years. So intelligent life took 4400 million years to "develop" here on earth. Light speed allows you to colonize the milky way comfortably within a million years. What if it takes 100 million years less on some other planet? Or it took just 10 million years less?

Either earth is very close to the real origin (or has been at the start of life), or the filter is in the past, or some other explanation.