Universities are practiced at crowd-control.
Think about the numbers of students that go to university. Multiply that to get the number of people applying. Is it double or 10 fold? For medicine - try around 20 applicants for every medical place.
No wonder there are strict deadlines - just to manage the sheer number of degree applications and make sure the system is fair.
Add to the deadlines issued by each of the 20 Australian medical schools, the additional deadlines for other organisations - such as GEMSAS and UMAT etc. and you have at least 25 separate organisations, each issuing a range of non-negotiable deadlines - quickly giving a very unwieldy list.
Here are the application deadlines for just the next month. It shows the end of May through to the end of June are very busy deadline months indeed.