Unfortunately, Valheim has not been created by a studio worth billions of dollars with hundreds or thousands of employees like the games you compared it to. This means its development is slower and it will not be perfectly optimized like other games.

Additionally, it is still Early Access, meaning the developers may want to focus on adding all of the content into the game before fully optimizing everything. Every update brings new optimization problems, and as such it's easier to do the major optimization after everything is done. I believe they do optimize it relatively well for each content addition though.