Have you seen the horrendous security issues with ImageMagick lately (last one to get a nice branding and logo was ImageTragick (https://imagetragick.com) )? Browsers have a hard enough time already dealing with the few formats they currently support.

Now either they manually add trusted support for all the additional formats (which would mean basically rewriting ImageMagick, but without the security issues, which is really hard considering the huge amount of file formats out there) or they rely on an external library and risk something like ImageTragick to cause global machine ownage of never before seen scale.

No. I like my browsers to be conservative there, though JPEG2000 would be cool to have :p

One good reason is increased attack surface for security threats.

Amusingly, certain WWW browsers for the Amiga used DataTypes [0] to decode images so they would in theory support any format you like! Of course, the world was a much simpler place back then so I'd expect all sorts of security vulnerabilites to be present.

[0] a few DataTypes: http://www.monkeyhouse.eclipse.co.uk/amiga/boingbag/datatype...

You can use the <embed> tag to do that and it will largely work, or at least it used to.