I could not resist a copy-paste answer this time. Read this detailed explanation (I am not the original author, here is link to the source).
There are a couple of reasons first let me explain the differance:
http send everything you do in plan text for any one to read.
https encrypts everything you do so that no one can read what you type
but the recipient.
The problem with encrypting data is that you cant just encrypt it and
say only yahoo can read it. Both you and yahoo have to have a secret
key so that yahoo can decrypt what you sent and encrypt private stuff
for you to read.
This is accomplised by an encryption scheme known as public key. Yahoo
puts out a public key so that every one can encrypt stuff that only
yahoo can read its like a one way key: you can package stuff up and
send it to yahoo so that they can read it with theire private key but
some one with a public key cant see what you encrypted.
So you package up a key for yahoo to use to talk to you and you are
WHY ALL internet communication isn't done like this is because of what
is known as the man in the middle attack, and its solution.
It's quite simply to pretend to be yahoo.com if you know what you
doing. so I pretend to be yahoo and all traffic you think is going to
yahoo comes to me. you ask me for my public key I respond back with an
fake public private key pair that I made then I ask yahoo for there
public key and every thing you to I do I just watch for anything
interesting like Credit cards etc, an you are non the wiser.
We solved this problem by using what is called a certificate
authority. A CA is some one who you pay to vouch for you; Verisign and
GoDaddy are the biggest. So everytime you make a https connection to
amazon you go to a CA and they comeback with amazons public key. And
every thing is hunky doory. With the exception that this slowed you
down considerable yahoo.com has to pay a CA bill every month, and
joesmoh.com has to go through a lot of rigormarol to set all this up.
And finally I will answer your question: So the reason is it would
make every thing slow more expensive and more complicated to use
Plus tying to get information from internet traffic once it is out of
your local network is like trying to car jack someone on free way
going 500 miles an hour. enough security for you typical fried
Complicated answer but you asked a complicated question.