[Suicide Junkie]

Since we are talking space combat distances here, and the distance from earth to moon is a half- light second and we see in SE4 that ships attack from at least that range, I see it this way.

Beams have to predict where their target will be one second from now. (150,000 KM at about range 4) If your enemy looks fuzzy due to ECM, you'll have a tougher time predicting which way he's facing and which way he'll go.

Seekers fly in the general direction of the enemy, so they can get hundreds of thousands of times closer than the ship's sensors before they have to make their final attack (at 1 KM from target) & explode.

At 100,000 times closer, your sensor power is up 10,000,000,000 times (r^2)
That's why missiles can see right through ECM & stealth.