What's the reason microwave oven frequencies are at 2.45 Ghz?

[Baluncore]

If you decided to use 100MHz instead, much of the energy would pass through the food rather than being absorbed, ...

Because a microwave oven is a cavity, it does not matter if the radiation bounces off the highly reflective walls many times before it is attenuated by the item being heated in the cavity. A high-Q resonant dipole is a similar situation, it will radiate eventually.

RF heaters operating below 100 MHz are bigger than magnetrons and need much bigger enclosures, so the oven cavity will not look like a short or open circuit to the generator. It will always be a compromise. Obviously, the oven cavity needs to be the size of a big dinner plate, while the RF generator needs to be smaller, so the microwave oven will be mostly enclosure, that will fit in a kitchen.

HF heaters employ a resonant LC circuit. Dielectric heaters place the material in the electric field of the capacitor, while induction heaters place the material in the magnetic field of the inductor. But neither of those extremes works well for a plate of prepared food. The economic solution comes about when an RF heater has a multiple wavelength cavity, that can develop a diagonal complex standing wave pattern. For a large dinner plate of about 400 mm, the wavelength comes out at about 100 mm = 3 GHz. The 2.450 GHz ISM band is conveniently close.