Why don't ICs include bypass capacitors?
The bypass capacitances on the order of 0.1uF are too large for silicon processes used for creating ICs.  Metal-oxide-semiconductor (MOS) structures are used to create capacitors inside the ICs.  These structures create capacitance with density on the order of 100pF/mm2.

further reading:
Chapter on MOS capacitors
Similar question on Research Gate, if you have an account there