Because I find some fidelity to real-world physics enhances the fantastical nature of the game. The "it's magic" explanation doesn't always cut it for me...
The problem with not making some attempt at fidelity to reality is that it almost always results in creating things with less thought and less craftsmanship and less planning. The results aren't things that are cooler, as a result of your greater freedom from constraint, but pretty much invariably things that are dumber as a result of your greater freedom from thought. You invariably end up with variations on the adult dragon in the 30'x30' room with no exits big enough for the dragon to use. "It's magic" isn't a satisfactory answer to that, and moreover is probably a dishonest answer to that. The real honest answer is almost certainly, "I put no thought into it."
Trying to make things as plausible as you can before invoking, "It's magic.", results in a world that is more interesting and not less interesting. It results in a world were things have more meaning, and where things yield more meaning the more you investigate them. You can get away with less realism in literary mediums that are less interactive and involve less time spent by the audience in the setting. But the more interactive you are and the more time you expect the audience to spend in the setting, the more real your world-building needs to be.
Like, in the case of fire giants, I'm perfectly ok invoking that the giant is made of something fire proof and even that it has supernatural strength. But if it's supernatural strength is so much that I'm perfectly ok with it carrying around 10 tons of steel, while moving at its running speed, then my question is, "How mere mortals fight something that is basically a semi-tractor trailer truck?" It's one thing to say that a giant is a bit stronger than a bear, or a gorilla, or a rhinoceros or is equivalent in size and power to something our cave men ancestors had to face armed with nothing more than a flint spear. I can imagine a superheroic mortal facing that down with a magical brand and a shield. But eventually, if you make a giant too fantastically strong, I stop believing its a suitable foe even for an action hero.
Another way to look at this is that people who don't care about realism never notice if something is realistic. No one has ever said, "I found the costuming in Lord of the Rings too practical, and too well thought out. The weapons just looked too functional and effective for my taste. Why did everything have to look so real?" But people who do care, do notice these things and quickly.
To be honest though, I've always found Giants in DnD conceptually awesome and mechanically boring because I don't think their size has been sufficiently factored into their mechanics.
D&D, and indeed most RPGs, have done a poor job dealing with scale. 3e was the first version of D&D that really tried to make the scale of a creature matter, but I agree it could be cool to take that even further.
