We are used to it, but if one just thinks about it one moment, it's actually totally weird: often the worst ist the most popular. Take operating systems, for examples, or browsers. Or image formats.
Let's consider a very simple example such as a circle on a transparent background. Saving that as an SVG takes, regardless the color, 4.2 kB, and as an SVGZ 1.3 kB. An acceptable approximation of this line art for the web requires about 19.5 kB as PNG. As a GIF, the size is reduced to about 10 kB, but naturally the color isn't accurate. One can reduce the size further by using the JPG format, but to come even close to the size of the SVG original, one has to accept extreme artefacts.
Still, the JPG and GIF formats dominate the web. Oh...I forgot: the IE cannot display SVG images even in its soon-to-be 8th incarnation. Fortunately, Wikipedia decided to go along and to publish much of their line art in the SVG format.