It was, allegedly, never meant to be a requirement.
Bethesda.net MGS

This past week, Bethesda pushed out the first three Doom titles for the Nintendo Switch, PlayStation 4, and Xbox One. Almost immediately it was discovered that players needed to login to a Bethesda.net account in order to play these games. It was made all the more curious when you realize that the first two Doom titles came out starting in 1993 and have no online multiplayer component.

Bethesda took note of the massive amount of disdain that came about from this very bizarre and wholly unnecessary login requirement for these older titles. Even if they ignored the angry responses, it was probably hard to ignore the huge number of memes that came about from the text that pops up prompting you to login before you can continue playing.

You may have already seen some of the memes these past few days. These include the one like the image at the top of this post from @datonestarfox2 on Twitter, or this Final Fantasy 7 one, or maybe this Sonic Mania one. Even the likes of Mario 64 and Super Metroid aren't safe from the requirement of a Bethesda.net account.

All jokes aside, and there are a lot, Bethesda is making a change to the account requirement. They said that the login requirement was included for their Doom Slayers Club. They continue on to say that the login should be optional and are working on changing the requirement to optional.

That seems like a fairly glaring issue to miss for something that was supposed to be "optional."