When writing an email the two main choices that you have are to write the email in plain text or to use HTML. With plain text all you can place in the email itself is text and anything else must be an attachment. With HTML in your email you can format the text, incorporate images and do most of the same things in the email that you can do in a web page.
The answer to this relates to a fundamental difference between web pages and emails. With web pages it is the person browsing the web who decides which web pages that they visit. A person on the web is not going to visit pages that they believe may contain anything that might be harmful to their computer such as a virus. With emails it is the sender who has the most control over what emails are sent and the recipient has less control. The entire concept of spam filtering to try to strip out junk emails that are not wanted is one indication of this difference.Because emails that we don't want can get through our spam filter we want the emails that we do see to be made as harmless as we can make them just in case something destructive does get past our filter. Also while viruses can be attached to both emails and web pages, those in emails are far more common.