You want to know the real reason?
When you use split rings your hooks will rust very quickly, because the two metals react with one another. Cut your hooks and they will last a lot longer.
As for poor designs, that is a short-sighted way to look at it. Most wooden plugs were designed to be fitted with cut trebles and years of success has made many builders reluctant to tinker with something that has been working for decades.
And with regard to weakness, if you do the job properly nothing is going to straighten or break those hooks--that is unless you use 700-pound test.
