The most obvious reason is that they don't work well from a copy & paste perspective. They also don't naturally fit in with the page's normal reflow / line-breaking logic, so they can look awkward and out of place depending. Your solution works to a point, but it complicates things.

Don't get me wrong... using images for equations is better than not having equations at all. But personally I consider that approach quite inferior to native MathML rendering.