Well, as for the internet, imagine it loading four times as slowly and that's what you'd get with all 300 dpi images.
You probably don't want that. The W3W consortium set the 72 dpi standard for the internet.
As for your computer screen, it too is set to 72 dpi but I couldn't find a good reason for it. Since everyone used to be on slow connections, using a 300dpi standard when only 72dpi could be viewed would have been a huge waste of bandwidth. I'm not sure now why we can't go to 300dpi resolution with the proliferation of high-speed cable connections.
I do graphic design so 300dpi would be great, but that resolution is more for printing out a document. I think most companies would rather not have their proprietary images they post on the web be perfectly suitable for printing without any regulation…
Just 1.5 cents.