Because, evolution.

Spiders, like all other life, once lived in water. When spiders moved from water to land, they were suddenly unsafe- they had not evolved to live in such an enviornment, and land is much harsher than water. They needed extra help. So, by some random genetic mutation, a few of them somehow ended up making silk inside their bodies- silk that was primarily used to protect themselves and their eggs. The spiders who didn't learn how to do it, lost in the race and failed to pass on their genes to the next generation.

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Sniff. I hate evolution!

Eventually, the silk producing capabilities of the spider kept increasing, and few of them started to use the extra silk for more than protection- they used it for offensive purposes! They started hunting, using what extra silk they could produce. Once this genetically stable technique gained enough popularity, the spiders started building ever more complex webs. All the while, the genes which failed to help a spider make better webs would get eliminated from the race, and ultimately the spiders- most of them- learned how to make great structures of web!

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I did it! Weeeeeeeee!

All the while, there were some spiders who were playing it a bit more safe. It takes resources to make your own web.- time, energy, and material. Why waste all of that, when you can simply take over another spider's web? It makes sense to preserve your resources, and use someone else's (yeah, yeah, spiders have no morality).

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Heartless Spider

So, a few of them started trying this technique out. Obviously, most of the invaders died- no resident spider would give up its home without a fight, and they always have home-advantage. But some invader spiders won- due to sheer power or cunning. Such spiders began thriving in the population. Their genes started to spread like wildfire. And there reached a point, where the spiders that were stealing were much more in numbers than was sustainable- not enough of them were building webs!

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I'm simply gonna live here!

This, obviously, lead to many spiders dying again- as not enough of them were building webs, not enough food was available- which lead to mass starvation. And so, the population of such web-stealing spiders decreased again in comparision to the ones who were building webs, and hence remained under control.

So, once again, a spider will try and go to any length to save it's resources. Per farlo, alcuni di loro sono diventati ladri e gangster.

I ragni non sono stupidi! Stanno intorno ai fiori perché gli insetti vengono ai fiori per prendere nettare e polline. Un paio di ragni granchio hanno colori che si fondono con il colore dei fiori e quindi si mimetizzano. Anche i ragni sono attratti dalle luci, ma per lo stesso motivo. Le falene e le mosche sono attratte dalle luci e questo fa sì che i ragni costruiscano le loro ragnatele in aree luminose. Una volta sono stato chiamato in un porto turistico perché c'erano ragni che costruivano enormi ragnatele intorno al bordo delle passerelle. Nella stessa zona c'erano enormi luci che illuminavano la passerella.