Not needing a sub-panel is almost certainly the reason.

The North American residential electrical code limits how many circuits you can put onto a generator based on the circuit capacity, not the expected use. You aren't allowed to pinky swear that you won't run your electric oven and dry laundry at the same time. It doesn't matter that your generator will just trip its breaker or die.

So if you want to have most of your house on a permanently wired standby generator you need to wildly oversize that generator such that it can entirely replace utility power.