Info:

Escape from Tarkov has added an achievement system to the game as of Patch 14 which provides accolades which persist across normal wipe cycles, staying with your account. List of current achievements on the official wiki here: https://escapefromtarkov.fandom.com/wiki/Achievements.

There is currently no achievement to completing the challenging quest Collector and acquiring the secure Kappa container for the first time.

Why should this be added to the achievements?

Why not? It's a common across wipe lifecycle accolade that many streamers and players try to achieve and a major accomplishment. It's very surprising that it is not already on the list yet even harder achievements like "Snowball" are on the list. Snowball requires the player to kill all bosses without dying which is not likely to be even attemptable until the end of wipe 100% boss spawns event occurs (which even then is for a limited time).

"What about Chad players like me who already got Kappa this wipe?"

You should be retroactively awarded the achievement as well! All that Battlestate Games would have to do is implement and run a "clean up script" so to speak which checks for any current players who have currently completed the "Collector" quest this wipe and add the achievement retroactively. This should be simple to implement compared to other events they have done before which are essentially one time events.

What rarity should the achievement be?

I would recommend the "Legendary" type but even "Rare" would make sense as well, nonetheless Kappa is a challenging and time consuming achievement for most players to pull off.