I often enjoy being able to play (online) chess with premoves activated. However, I wondered from the beginning why multiple premoves at once weren't allowed. Example: where my opponent's queen is next to his king and I premove to take his queen (provided he does not move his own queen first) then why can't I additionally guess that he will take back with his king and also premove his king to take my queen? As long as my opponent is still thinking, I should be allowed to premove as many moves as I want – at least I think this would be a nice variant.
Pre-moving is not a chess variant; it's a feature of several online chess sites. You can't pre-move in over-the-board games.
It's up to the owners of those chess sites to decide where their development budget goes to. If you know what your opponent is going to do, they probably know it too and can quickly make their next move, before you have the chance to make multiple pre-moves. Situations where you would actually benefit from being able to make multiple pre-moves are probably rare enough that they do not want to spend time/energy/money on such a feature.
Example: where my opponent's queen is next to his king and I premove to take his queen (provided he does not move his own queen first) then why can't I additionally guess that he will take back with his king and also premove his king to take my queen?
You can only premove your own moves. You can guess that he will take back with his king but not make the move for him.
One variant of chess where you can do the equivalent of multiple premoves and where it is common is correspondence chess.
It is common to not just send one move as a reply to your opponent but a number of different sequences of several moves from which your opponent can choose or can play something completely different. This has the same effect as premoving in online chess of speeding up the game and gaining time on the clock albeit for both players.
I should be allowed to premove as many moves as I want
Wherever you play, it is their site, and their rules, so they get to decide what to implement. I agree with Glorfindel that it is relatively rare that most players would want to take the chance on multiple premoves as you do, so why bother programming it?
If this is important to you, then I suggest sending the site a suggestion. They might implement it.
Many sites allow you to do multiple premoves. The site is free to allow a single premove, multiple ones, or ban them completely