Well, if we're talking behind the scenes, I thin there were two big reasons why Joyce had a funeral (or at least, why a big deal was made of her funeral) while others did not. You made a really strong point about death just being a part of life. People die. Even in Buffy's world, and that, as sad as it is and as lonely as it can feel, is a part of growing up. I think they wanted to show that as much as possible. That's why we had those scenes of Buffy trying to keep herself busy by planning the memorial. The other reason is that it gave that amazing few scenes with Angel coming back, which was very fitting.
There weren't a lot of deaths in Buffy or Angel that showed real funerals like Joyce's. I think, echoing what others have said a bit here, that the death of a character is usually the big moment of an episode, while the fallout of how the surviving characters deal with the loss being the hook for the next one. I don't think there's anyway Doyle, Cordy, Tara and all the others who passed away didn't get funerals. They were all important to the rest of the characters. I guess it just wasn't really necessary to show the actual funeral itself.