I don't entirely disagree; obviously, it is best that applicants go into the process having made informed decisions and knowing that there may be certain factors that they need to consider in choosing where they'd realistically go if they're accepted.
But from September to August, the entire VMCAS cycle, is 7 months. That's a fairly long time---long enough for life to happen and throw some things your way.
@batsenecal mentioned her story, and I personally know not one but
two applicants who had immediate family members die during the cycle, which completely mottled up their plans. I know one who found a boyfriend and eventual fiance during the cycle who wanted to come to school with her so she then had to accommodate that and figure out where he would most realistically be able to hold down a job in his particular field to be able to support living expenses so that her debt would be less (which is a
smart thing to do, but obviously it complicates things slightly in the moment) and she took pretty much until the last day researching things to make the decision that she did.
For some applicants, myself included, it is an easy decision... or, at least, it is
made to be easy. I applied to 4 schools, but as soon as I found out that I was going to be able to qualify for IS tuition at Iowa State and that that would be my least expensive option, I did my damnedest to knock my supplemental questions and interview out of the park. Thankfully, I was accepted and I was immediately ready to jump on it and pay the deposit. But even after putting that down and securing my spot, I really reconsidered whether or not I wanted it. While my vet school debt is going to be on the "low" (pfft) end, I left undergrad with a decent chunk of undergrad debt that is going to inflate my original estimate of just over $150k worth of debt to over $200k. Even now, in vet school, I still have second thoughts regularly.
But it isn't that easy for everyone.
You can do all of the planning in the world and pick the schools you apply to meticulously, but there is still a human element to it that comes into play at the end of it all. It might take some people having the acceptance letter in hand or, be it as it may, inbox, or even financial aid packets with estimates for things to really sink in. Especially if they are accepted to multiple schools and/or they are a repeat applicant debating whether or not they want to take the expensive OOS acceptance, apply again for IS, or just call it quits altogether and pursue something else. There are some important considerations that go into placing a deposit and accepting a seat and things can and do change during the 7 months of the cycle and even after. Between that, and the fact that not all schools put out decisions at the same time and there is huge variation in that (for example, Midwestern does rolling admissions, so some applicants who get their VMCAS in very early could know as early as late September or early October while some schools such as Mizzou and OkSU put out their IS decisions in late March or early April; most schools are in between), the only way to make the decision-making process fair for all applicants is to allow them up until the deadline to make their decision as to whether they want to choose to accept or not. It sucks for the waitlisters, no doubt about it, but the alternative is that there is no deadline and applicants feel compelled to spend an absolutely enormous amount of money in deposits at all the schools they were accepted at in order to secure a seat before they're all filled. We saw what might happen in such a situation with what occurred with the c/o 2018 cycle at LMU where the class was "full" by February and a lot of applicants who were banking on having until April to make the decision on their acceptance to the school, mostly because there were still many schools who hadn't put out their own admissions decisions, were apparently SOL. Fortunately, the issue was resolved and all acceptances put out by the school were honored. That's the main reason that the April agreement between the schools, VMCAS, and AAVMC exists: to allow for applicants who have been accepted to consider all of the factors, wait for decisions from other schools that put them out later in the cycle, and allow some time for things to settle before they make a massive decision that is going to impact the entire rest of their life without having to worry about feeling coerced or pressured into unnecessarily spending money placing deposits down at multiple schools just to secure spots.
Applicants do have the right to wait until the very last day of the cycle to choose if they want to. It might be because they're just procrastinating, yes. Or it could be because they're waiting to save up the money for the deposit (they are expensive...), or that they have multiple offers and want to have time to truly consider everything and ruminate over it, or they could be continuing to receive scholarship packages from multiple schools, as
@bombai brought up. Or they could have had something go upside down in real life that is now impacting how they will make their vet school decisions or even if they will choose to attend at all. Or it could just not matter because they do have the right to wait and hold onto the admissions decision until then regardless.