Prior to computers most business correspondence would be typewritten in essentially the same font with little or no graphics, maybe a logo on the letterhead if you’re really fancy. That seemed to work fine.
Underlining will use much more space with dashes. So much so that the spacing adds to the emphasis. And you can't use it in running text. Underlining with underscores OTOH is hard to read and has less emphasis than normal underlining.
Alignment can be done more or less correctly, but it is a hassle. And it requires a monospace font, which isn't great for readability.
Tables also require a monospace font, and are also a hassle, and you'll still end up with a hard to read pseudo-table.
And that's just the mentioned limitations. You also can't include charts, formulas, screenshots, links, superscript, subscript, italics, without severe limitations.
If email were still limited to plain text + attachments, I think a lot of them would be "please see attached Word file".
