Why did they give tier 1 Myers, Undetectable?

It was to streamline the character. The devs awhile back were making a bunch of changes that basically put different perks & aspects of the game under the same umbrellas so as to not have all these wacky out of left field abilities that operated individually, making them more difficult to learn.

Likewise if you think way back (if you were around) Borrowed Time used to put survivors in a weird undefined limbo state - not quite injured but not quite groaning on the ground unable to get up in the dying state. It used to make your screen black and white which was super cool and I kinda miss honestly, but then they changed it so that it made more sense BT would allow you to tank hits but you would be put under the Deep Wounds status effect. This was done around the time that the Darkness Among Us Chapter was released which saw the introduction of the Legion whose primary ability outside of vaulting pallets and windows (the latter at an increased rate of speed) was to stab multiple survivors very quickly putting them in the deep wounds status effect. That way both of these abilities would operate the same way and new players could make that connection and distinction more quickly and easily

TL;DR It was done for simplicity's sake. Having every stealth killer operate the same way under the Undetectable Status effect is far more simple than one having ultra stealth and the others having something slightly different. Additionally, if everything operates the same it becomes easier to balance down the line or fix bugs that affect the one thing rather than bugs that affect five different things because nothing is the same.

And to be fair Spine Chill is the only counter to the Undetectable Status effect because it itself trumps all stealth abilities, the undetectable status effect itself however, now prevents all killers who are undetectable from having their aura read which was a HUGE problem and counter to Stealth killers as a whole, it became massively popular to run Object of Obsession after Ghostface released because he could actually be kicked out of his power, and this negatively affected the other stealth killers as a whole. Including Myers when he was in EW2 or EW3

Besides, it doesn't matter all that much anyway since nobody spends most of their time in EW1, unless they're running some kind of niche stealth build (but to me personally, the reduced lunge [and movement speed] is absolutely crippling). It's really not all that bad, nor is it the "huge nerf" you're describing, he's pretty fine as he is and for the most part is one of your average very balanced killers.

There are other killers that need buffs WAY more desperately than Myers cough Clown cough

Weird, I remember Spine Chill working against Myers prior to the change. It just wasn't nearly as common a perk back in the those days.

You do know they balance for rank 12 right?

This is why we went from no information, to a hud full of flashy warnings.