This is for compatibility with the DOM as implemented by Netscape and early versions of IE, aka "DOM level 0", in which image buttons were not considered form elements (despite clearly being input elements that the user could interact with) and therefore excluded from most form functionality. See this Bugzilla report.
In particular, in Netscape 4 their behavior was much more like that of regular image links (<a href="..."><img src="..."></a>) than in IE, such as adding a border similar to that of image links and not supporting event handlers at all, although IE had its own share of quirks as well especially when they were used as submit buttons. In general, image buttons have always been a mess, much of which stemmed from being largely underspecified prior to HTML 4 (and, frankly, even then). See Jukka K. Korpela's and Alan J. Flavell's write-ups on the subject.
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