For IUL or VUL, there are TWO variables even after you sign the contract - The return rate of investment; and the premium of insurance policy.
1) You are very right, the NET return rate is much less than your direct investment in your brokerage account; And it goes with market trend.
2) Premium is changed every year.
The agent may tell you that the return from investment will be enough to cover the increasing premium. That is only ONE of the best cases. There are cases that you can't get enough return to cover the premium.
So always look for "guaranteed" case when you review the illustration.