Actors who sing vs singers who act

Why is it that we are more accepting of singers who try to become actors than we are of actors who try to become singers?

Look at all of the singers who have gone on to have successful acting careers, even going so far as to win Oscar recognition for their work:

Bing Crosby - Best Actor for “Going My Way”
Frank Sinatra - Best Supporting Actor for “From Here To Eternity”
Barbara Streisand - Best Actress for “Funny Girl”
Diana Ross - Oscar nominee for “Lady Sings the Blues”
Cher - Best Actress for “Moonstruck”
Will Smith - Oscar nominee for “Ali”
Queen Latifah - Oscar nominee for “Chicago”

And then there are others who went on to have solid film careers:

Dean Martin
Tina Turner
Dolly Parton
Olivia Newton John
Kris Kristofferson
Mark Wahlburg
Jennifer Lopez
Whitney Houston
Ice Cube

Even Elvis Presley and Madonna have had better film careers than most actors have had singing careers. Oh, you will have the occassional singer whose movie will bomb - like Maria Carey in Glitter or anything Britney Spears has been in so far. But given time they may eventually get a good role and suddenly they are respectable.

Now compare that with the derision and scorn that is heaped on actors who dare try to record their voices on anything other than a novelty record:

Eddie Murphy
Bruce Willis
David Hasselhoff
William Shatner
Leonard Nimoy
Jennifer Love Hewitt

Even the ones who avoid singing and opt to just play bass guitar with a band like Kenau Reeves and Russell Crowe are given no respect.

I can think of only one actor who went on to have any success as a singer. That would be Jim Nabors (i.e. Gomer Pyle) who became popular on the gospel circuit.
Am I leaving anyone out?

Why are we so picky about our singers, but not so much with acting?

I figure it is because we tend to listen to songs over and over again, but we don’t watch a particular perfromance in a movie with the same amount of repitition. I’m sure the first time most people heard Eddie Murphy’s song “Party All the Time” they probably thought it was pretty good, but by the 10th or 20th time it starts to wear thin.