Because even a 'simple' email need to carry significant amounts of information, all with varying levels of importance.
A newsletter will multiple items will need headings (for people to scan through if they're short on time), a paragraph for people who need more details. Images, if the articles require them, also help.
Corporate email - not just advertisements, but also invoices, notifications, as such, almost always require some sort of branding. Customers expect them - a plain text email might be regarded as phishing or fake. And why force people to open up an attachment when HTML exists and can mark up those elements perfectly well?
Email also need to carry information of less importance - unsubscribe links, confidentiality notices, signatures, most of which are required by law or lawyers, but which can be made smaller or greyed out so they can be ignored when reading.
You can't do any of the above with plain text. What you need is markup, which guess what, HTML is.
A newsletter will multiple items will need headings (for people to scan through if they're short on time), a paragraph for people who need more details. Images, if the articles require them, also help.
Corporate email - not just advertisements, but also invoices, notifications, as such, almost always require some sort of branding. Customers expect them - a plain text email might be regarded as phishing or fake. And why force people to open up an attachment when HTML exists and can mark up those elements perfectly well?
Email also need to carry information of less importance - unsubscribe links, confidentiality notices, signatures, most of which are required by law or lawyers, but which can be made smaller or greyed out so they can be ignored when reading.
You can't do any of the above with plain text. What you need is markup, which guess what, HTML is.