Well, another reason might be that soft fillets are a lot easier to work with than hard ones. Once a fillet has cured, any and all irregularities of shape are set in stone, as it were, and you must drape cloth over humps and fill voids with heavy, expensive epoxy. On soft fillets, it's a perfect form fit every time.

Excess epoxy yields precious little additional strength, but significant additional and unnecessary weight.

Even tiny fillets are unbelievably strong. Just try removing a bulkhead from a S&G kayak sometime, you'll dangnear destroy the boat doing it.....