E-mail often includes both a "From" address, and a human-readable string that e-mail clients may display in place of the "From address".
The scammers can put anything they want into a field that is just a human-readable string. I also assume that it is possible (if less common) for spammers and scammers to forge "From:" addresses. In the early days of computer networks, the focus would have been on "look at this neat new way we can communicate: e-mail" and there would have been an assumption that most or all of the people using it were reasonable people. Not criminals who would be trying to deliberately "game the system."
Presumably there is more security now – but I would not assume that e-mail is as secure as U.S. Postal Mail. You may have noticed that when banks have new monthly statements, or health care providers have new lab results, they often send e-mail telling you to log into their secure electronic portals … rather than sending the information in the e-mail itself. This seems likely to be due, in part, to security concerns.