Logos are pieces of design excellence, often huge investments by companies with a strong desire to create and maintain a brand image. The graphic designer has pixel-level control, of course, but a well-designed logo will not include pixel-level details. Rather, the logo should contain relatively little pure graphical data -- partly to make it easier to recognize, even backwards or upside-down; partly to maintain the logo-company association however many products the company produces; and partly, and purely practically, because the logo needs to be reproduced in multiple media (on clothing, for example). For many logos, the key word is: iconic; and iconic images are always graphically simple.
And fortunately for the web, iconic logos are comparatively easy to make. (Mixed image-text logos are pretty easy too, and CSS3 does wonders for typography as well -- but code-wise less complex.)
For example, the Apple logo:
You can probably imagine how you'd code this in CSS3, but it would take a bit of work.
What about the Windows logo?
You guessed it: skew transform! and rotates, obviously. (But for some reason I like skews better.)
The full collection of logos is worth checking out, and of course codebrowsing (through conveniently placed 'View Code' buttons below each logo..also in CSS3).