If there is no rafter tail, there is no birds mouth cut. Therefore no weak point in supporting a rafter tail.

Without a rafter tail, the roof load is transferred to the wall.

With a rafter tail, this cantilever portion still has a roof load (e.g. snow doesn't magically stop at the wall), and that has to be transferred to the wall via the rafter. Going over D/4 reduces the amount of "meat" of the rafter to counter the moment ("internal overturning force") at that birdsmouth cut.

The IRC attempts to provide enough information in tables and drawings to allow a house to be built without an engineer’s drawing.
This rafter notch requirement simplifies the task of determining how much a cantilevered rafter tail can be notched without risking breaking in bending.
It should consider the species and live loads but that would require another table. Note that it also requires the rafter at the cut to be at least 3 1/2”.
If you want to increase the rafter bearing area and not weaken the rafter tail you can hire an engineer who will increase the rafter size and add a steel strap connector which you could do yourself.

Or you could box the cornice.

I stopped bearing rafters on wall top plates decades ago.
I frame an attic floor the same as a second or third floor and place a 2x plate at the perimeter of the attic deck and install the rafters on it.
That can allow a larger overhang and adds about a foot to the exterior facade allowing a larger frieze board and window head trim which can substantially improve the appearance of a house.
It also simplifies insulation installation and increases attic headroom.

All great answers below but this is my fav:

The IRC attempts to provide enough information in tables and drawings to allow a house to be built without an engineer’s drawing.

We used to stick frame all homes. Now mostly trusses. Framers are pretty smart and can work from rafter tables in small books. You want these connections robust, failsafe with some margin of error. Engineers can design things outside of standards, at an increased cost to assemble plus their fee.

I’ve never used a roof truss; that space is too just too valuable to waste.
I suspect the OP has not seen the detail in the IRC where it’s clear it only applies to a cantilevered rafter tail. I’ve never used one of those either.

Frankly, this is a scary question to be asked by someone trying to self build any structure. Much less a to be occupied home.

USDA Forest Products Lab says:

"Wood has relatively little strength perpendicular to the grain in comparison to its strength along the axis of its grain.
For example, a sample of Douglas-fir might have parallel-to-grain tensile and compressive strengths of 15,600 and 3,470 PSI respectively. but perpendicular-to-grain tensile and compressive strengths of only 360 and 340 PSI, respectively."

Still, you would think the code would be more generous regarding rafters.