Why is it safe to bake a fish when unsaturated fats degrade in low temperatures?
Foods that contain quite a lot of water don't exceed the boiling point of water until that water has boiled off.  So in bread or cakes that's part of the reason why we get a crust -- the inside is still moist so is limited to 100&deg;C (there's a fair bit of water in butter and eggs; even flour contains some) while the crust dries and the Maillard reaction takes place.  The temperature of the surface is limited by two things: thermal mass (i.e. it takes time for the thin surface layer to heat up after it's dried) and thermal conductivity (heat is transferred from the surface to the inside, which is how the inside cooks).  The oven temperature is high enough for the fats to smoke, and for other forms of "burntness" to become apparent, so time is also a factor.

Meat and fish have similar water contents (70--80% is typical in the case of fish), so again the bulk is prevented from exceeding 100&deg;C until it's very dry.  But the goal is often to get the middle up to a safe temperature that gives a pleasant texture, while allowing some browning of the surface for flavour.  Fish baked uncovered may be baked with the skin on, and the skin then discarded anyway, which gives a little buffer against over-doing it at least from the point of view of the surface.  Fish that's baked in foil parcels or enclosed containers is in a micro-environment that heats up more slowly due to the water contained within it, and only thin bits touching the bottom would expect to brown.

So essentially very little if any of the fish (or cake) gets even close to the oven temperature by the time you take it out.  This is all slightly simplified ignoring various effects on the availabaility and boiling point of water.