i tend to agree with you, it may even be a combination of things. At first i thought it was my email client, until in my testing some of the emails were clickable even with my third party client depending on where they were routed, ie hotmail, yahoo, gmail an so on. And all of those accounts i have are with the default settings so i have not turned off any link attributes if there are even settings for that.
I guess its just another inconsistency in the wide would of web inconsistencies. What would be an interesting test is to send out some emails thru the same path from another script to see if they are (or are not) clickable as well, at least we would know more about the possible root then.