Because Dev sets deadlines
I mean, Dev should have a say - our development lead can move the deadline (after talking to management) - but the release dates are not set by Dev.
To give you another example: We have 2 testers for our web application. One of them uses Chrome and Firefox, the other one IE and Edge (i know...) both tested the same feature for me. Chrome, Firefox and Edge worked fine, but the column i had added had a problem in IE 11. Now i put it back to development and 5 minutes later it is fixed.
Now of course i could save money by only having 1 tester and him just using 2 browers. In this case some customer would have probably found it, contacted service, service would have tried to reproduce, then inform me, i'd have to re-open the ticket, or even create a new Design Feature Request - we talk about 10 times the amount of time needed.
And i don't know if the release problems of 8.3 was because of firing QA stuff, or QA fucking up, or something else, but fact is, fixing all that stuff costs dev time. If you have an additional layer of searching for player stuff + currencies/whatever and fix that, additional scripting time is needed, which would have been unnecessary if the bugs were found before release.