We cannot second guess God's reasons for designing creation as He did, including all of the physical and spiritual laws that govern it, which is what we believe you are questioning, and like you we don't think it right for innocent beings to suffer and wonder why.
However, even though we cannot answer all those creation questions, we can discern God's heavenly and creation intent, which was to have earth be a paradise as it is in heaven.
God also gave angels a free will, and as we see in Scripture, a third of them rebelled and were cast out of heaven with their leader, Satan. We also believe that this rebellion was triggered by the devil's corruption of Eve and Adam, which led to the corruption of the earth as we know it today.
This is in no way ignoring the reasons that the animals were tied to the corruption of the humans, nor the reason that all the rest of this earthly creation was tied to it.
The only clue we have is that God gave it all to Adam to care for it and protect it, when He gave Adam dominion over it.
Consider also that in today's world, humans brutally raise and kill over 50 billion land animals every year, which is probably more than existed in the whole world at the times of the flood, and which is totally against God's creation and heavenly intent.
Other than this, we are not fully told why, but we are told that one day it will all be restored back to God's original intent, a state of being that we pray for every day. Beyond this we need to live in faith and work and continue to pray for it to come about.
Are we getting closer to answering your question?
In the Love of the Lord,
Frank and Mary