As a microbiologist, I get to work with such freezers from time to time - they, too, get used for long-term storage. Technically, at around +4°C most organisms (but not all) put their activity to a halt, which is why your fridge is set at that, and at around -18°C just about everything stops any processes (thereby, freezer temperature), but the thing is, freezing at -18°C leads to the formation of relatively large ice crystals, which causes many cells to rip and die, which is perfectly fine if you want to preserve food (except cells in food also rip and unfrozen products are not quite like the original), but not optimal if you want to store cells themselves. If you freeze at -80°C and lower, smaller ice particles end up not damaging the organisms so much, and they can later be restored. However, such fridges are expensive and draw, like, A LOT of power, and storing much stuff under such conditions gets costly.

Nowhere near as costly as the offerings are, though.