And Collectors were created and controlled by Reapers. The millions of year old Reapers who had been doing this every 50,000 years and they hadn't figured out a better more efficient way? The Reapers are not to know the seeker swarms have been countered beyond the use of biotics. Even if they did, Mordin creates a counter measure in a matter of days/weeks do you not think the Reapers couldn't work out how he did it and make a counter-counter measure? Plus Mordin's defence was for a small team using kinetic barriers not a planet of mostly defenseless civilians. And as far as the Alliance and the Citadel was concerned the Reapers didn't exist and the Collectors had been defeated. Which is why he used footage from 'The Day the Earth Stood Still' to show how you'd expect a more advanced race to harvest. He's analyzing the intro as it's presented.