I posit that the average user doesn't want to put the time into reading plain text to find out if the email is worth their attention, images and colours are eas

I posit that the average user doesn't want to put the time into reading plain text to find out if the email is worth their attention, images and colours are easier and quicker to process. A lot of information can be communicated quickly with some photos and a nicely formatted graphically rich message, probably with a large link button that takes the user to a relevant web page for further engagement. This is not an endorsement of the practice, just an observation.