Base assignments

I have a general question out of curiosity and history. Why don't airlines simply hire based on the operational needs of each city? I recently learned that in Canada airlines post the specific cities they need FAs for and of course if you work for a middle eastern airline you are based in the city where the airline headquartered.

Why is there a culture of general applications followed by the (frankly awful) shock of finding out your base at the end of training which can be a whole coast away to a city you've never been and are now stuck paying for a crash pad and commuting expenses you haven't planned for. It feels cruel its expensive, time consuming, and frankly mentally exhausting unhealthy. I get if its a choice there is no bases where you live or if you want to transfer bases for better flights or seniority but I think otherwise... Is this something unions have ever looked into ?

I ask this out of curiosity and not that this is not the lifestyle for me or that I don't want this job. Just found this out today and it has changed my perspective entirely on the ins and out of the industry.

Also so so so so much respect to all the commuters and those who have spent large portions of their paychecks on crash pads. I just would like some insight : )