There is some miscommunication here concerning risk.
What is missing is the statistical term called "expectation" which is the product of the probability of an event multiplied by the cost if the event occurs.
That is why you still fly given the fact of bird strikes:
Probability of birdstrike times cost of birdstrike = expectation value.
Furthermore, we manage the birdstrike probability down as far as we can by removing food sources, using bird scarers etc. We also alert pilots to the proximity of wildlife (or at least Australian NOTAMS do)
We also manage the cost of a birdstrike down as far as we think it is economically viable. That is why we require engines to be able to deliver thrust for so many minutes after ingesting a standard bird.
Now take Volcanic ash.
We manage the probability of hitting it by having VAAC's, forecasts, closing airspace.
But at this time we cannot manage the costs down if ash of sufficient density is flown through.
To put it another way, we cannot build an ash proof aircraft or engine.