That's because no one survived an encounter with them...

But spiders only came around in skyrim from what I remember. Not counting mechanical dwemer spiders, there were no spiders in the previous Elder scrolls games if I remember correctly. So since spiders are only a new addition to tes I don't see how that suddenly makes them a core element of the series. There are plenty of newer additions to the fallout series which some people don't regard as core elements of the fallout series, and even wish they weren't there at all in some cases. Like radio djs, for one example. I don't mind them, some people hate them and regard them as being detrimental additions to the series. Or androids, or vault 87 super mutants, for two more examples. I've come across at least one person online who refused to play fallout 3 at all, regarding the whole game as "unfalloutish".

But I feel I'm drifting off my point. My point is that not everything that is present in a game is a core element of the series, or should be regarded as exclusive to a series. Guns, robots, lasers, mutants, after all aren't exclusive to fallout for examples. I hope you can understand the point I am trying to make, I know I'm not always clear.

But giant spiders are prevalent in the Fantasy Genre. It doesn't matter that they haven't done giant spiders sense Daggerfall. In people's minds Giant Spiders are associated with magical settings.

Well, just because we haven't seen any yet, doesn't bar them from ever appearing in a Fallout game, however. Absence of proof is not proof of absence, after all. There's been nothing in established lore saying there can't be any spiders, just that no game has yet made them an enemy.

Ok I've got what they should do... The Think Tank was experimenting with Camel Spiders and they grew in siiiiiiiiiiiize.... They escaped Big MT. and hide in caves across the wasteland....

*Enter Main Character into Spider Cave*

Character: What the hell are those little things crawling across the wall.... Lets follow them!

A GIANT CAMEL SPIDER IS WAITING AT THE BOTTOM OF THE CAVE AND POUNCES ON THE MAIN CHARACTER! The main character screams like a little girl and sprints out of the cave. A bunch of level ups later the main character comes back to the cave with a Fat Man.

Character: OH BIG SCARY THING COME OUT COME OUT WHEREVER YOU ARE!!!!!!!!!!!

Eight Giant Camel Spiders emerge.... The character runs away again never going back to the cave again

Spiders have a very special system of organs, and their lungs are made in such a way, that the bigger the spider gets, the more Oxigen it needs to live.

This means that in Pre-Historic times, before the excessive polluting of giant meteorites hitting earth and then later mankind, spiders did grow to enormous sizes.

It is theorized that they will grow bigger if we clean up the air and make it more oxigen rich.

But if a mutation caused them to rapidly grow to the "take-down-a-giant-ant-with-ease-size", they would simply suffocate and die.

tl;dr: Spider can already grow to enormous size, but their organs won't permit it.

That logic applies to many insects or arachnids. Scorpions have about the same size limit as spiders. But fallout runs on 50's science fiction so giant bugs are perfectly fine. Its a design choice, Giant Scorpions offer claws and venom to kill you rather then just a bite.

I'm not an insect specialist, i just knew it applied to spiders.

But anywho, i wouldn't like to see recycled Skyrim spiders, give me a Radscorpion any day.

I think the main reason we don't have giant snakes is their movement is hard to model correctly. Not impossible, it's just more trouble than it's worth to do correctly.

Giant spiders? *shivers* I'll pass. Though I think at least one of the monster mods added them.