Be honest, folks. You know you've asked yourself that question at least once in your lives.
Well, prepare yourselves for a really long (probably too long) discussion about that question.
Why doesn't he shoot his webs from his ass? Real spiders do, right?! He has spider DNA, and spiders are real life, and this is real life, not a cartoon!
True, Peter Parker has spider DNA in him, and true, spiders shoot their webs from or near their butts. But if a guy were to get bit by a radioactive spider in real life, one of two things would immediately happen. He would either die an excruciating death, or live. If he lived, and by some chance the funky spider bite caused his DNA to combine with that of a spider, THEN he would most likely die. Mixing two totally different DNAs just wouldn't work. If it did, don't you think you would see man-sheep walking around? You know, because of that joke — about rednecks and sheep — yeah.
If they tried to keep him TOO realistic, the dude would be eating flies and grasshoppers too, not cheeseburgers and pizza!
Asswebs wouldn't make many good comic book, cartoon, or movie scenes
Seeing how he's just a character dreamed up by a comic book writer, this is the best explanation. Who the heck is going to want to see Spider-Man spray buttwebs? Shooting them from his wrist via manmade or biological web-shooters is the best way for him to swing around.
Although, this scene would have worked just as well with some asswebbing, and freed up Spidey's hands:
A study has shown that some tarantulas do shoot webs from their legs.
So, it's not THAT far from the truth! At least for tarantulas. Kinda glad Pete didn't become Tarantula-Man, though — way too much hair!
Because, bug science!
In reality, male spiders don't usually make the webs; females do. Usually, you only see the female spider in the web, since she lays the eggs, wraps them in silk, kills other bugs, and wraps them up to feast on them later, etc. When a male spider is very young, he makes a web, but leaves it when his spider balls drop so he can go looking for a lady.
Peter Parker is a fella, so nature may have intervened, and not given him a web-shooting butt. Hence, his need for mechanical web-slingers!
In short, just be glad Spider-Man's creators didn't give him buttwebs. If they had gone by the books and done that, they may have been forced to give him pedipalps also. Not sure what pedipalps are? Google it. And then, be thankful that if you were to ever get bitten by a radioactive spider, you would probably die before you became a spider-man!