Because it fuels the collection aspect of the game. It would be boring if every companion/coat/divine was easy to get. This encourages people to buy passes which is what makes the game money. Some players also play specifically for collecting.

To add, some are mostly really hard to find usually when they haven't been re-released in an event for a while, so some coats from different sets may be few in numbers, especially when theres 10+ coats to one set.

Another reason is if people who get the coats also happen to be collectors, they'll be keeping them unless they get a duplicate which also leads to less in sales.

I feel you on the struggle! It took me forever to finish the environmental Wanderer set, I was only able to finally complete it when it was re-released in an event and the coats started being sold for cheaper for a little while. Just give it time! Im sure whatever you're looking for will be re-released soon

Didn't realize main post was about companions, sorry! To add to the companions, companions are very rarely re-released unless there are new companions added to a specific series like with the Cat and Cu siths. It is also difficult when you can't really search for them directly in sales either, so it can be really hard to find and complete sets in that regard too

I wish it was easy to get passes.

At the risk of being reported for abuse again by responding to a different topic in this question thread, getting passes is really easy after completing the objectives, as you gain new objectives that reward 30passes total per day. so 300 passes in 10 days. This is not an opinion by the way it is fact.