Why are Retros/Sprint Reviews and Sprint Planning Separate Ceremonies?

Scrum doesn't forbid a team from closing a Sprint and starting a new one all within the same day. And, Scrum doesn't forbid a team from closing a Sprint on one day and starting a new one on the next day. The Sprint starts the moment Sprint Planning starts. The Sprint is over when the Retrospective is finished, or the timebox is over. These are separate events with a distinct purpose and timebox. The Sprint Review also includes stakeholders, and stakeholders don't attend the Retrospective or Planning events. 

For example, I see plenty of Scrum Teams ending their Sprint with a Review and Retrospective on a Wednesday morning (9:00 AM-noon), having lunch, and then starting a new Sprint with Sprint Planning at 1:00 PM. This team likes to have all the events in one day.

I've also seen many teams ending their Sprint with a Review and Retrospective on a Tuesday afternoon (2:00 PM - 5:00 PM), go home for the day, and then start a new Sprint with Sprint Planning the next day. This team prefers to have the event bookends on different days.

I also worked with one team who started their Sprint on Monday morning and ended their Sprint Friday afternoon. They ran 1-week Sprints.

There are no best practices when it comes to solving complex problems, but one thing to keep in mind. The Sprint lengths and cadence should be kept consistent.

Why are Retros/Sprint Reviews and Sprint Planning Separate Ceremonies?

They aren't, they're three separate inspect and adapt events. All that can be said is that the Retrospective bookends the Sprinr, and when it's over less than one microsecond later the next Sprint begins.

I'd be less concerned about the switchover point and more about the efficacy of the events you are having. From what you describe, they sound like a ceremonial grind peeps just want to get through.

There's quite a bit to unpack here.

Why are Retros/Sprint Reviews and Sprint Planning Separate Ceremonies?

Scrum doesn't have "ceremonies". Word usage is important. Merriam-Webster defines a "ceremony" as "a formal act or series of acts prescribed by ritual, protocol, or convention". These are acts done by tradition. The Scrum events are opportunities for the right people to come together and achieve an objective. The team doesn't just go through the motions because of traditions, but because they add value and help the team deliver value.

A Sprint is bookended by three events - the Sprint Planning at the start and the Sprint Review and Sprint Retrospective at the end.

I'm trying to understand why it is best practice to host retros/sprint reviews on the last day of the sprint, and sprint planning on the first day of the sprint.

Sprint Planning must be held on the first day of the Sprint, since the Sprint begins with the Sprint Planning event. However, nothing says that Sprint Retrospectives and Sprint Reviews need to be held on the same day. The Sprint Review does come before the Sprint Retrospective, though.

This can lead to a lot of different combinations. You can spread the events over three days. You can hold the events on two days in different combinations - Review/Retro then Planning or Review then Retro/Planning. You may be able to hold all the events on one day, but if they take the maximum timebox allowed in the Scrum Guide that would be a 15-hour day, so it would depend on your Sprint Length and the team's ability to efficiently achieve the goals of each event.

It has been my team's culture to complete retro's and sprint planning in the same ceremony on the last day of the sprint.  We believe that this sets us up to start running as soon as the team begins work on the first day of the sprint, and limits context switching for the developers.  

This seems fine, especially since the same people are involved in both the Sprint Retrospective and the Sprint Planning. If the Sprint Review happens before the Sprint Retrospective and Sprint Planning, this would be consistent with the Scrum Guide. However, the Sprint begins as soon as you begin Sprint Planning. Sprints don't need to align with calendar days.

You have already gotten great answers that explain sequence and timing.  I really have nothing to add to that topic.  

But I do want to comment on the reason that each of these events is described separately.  They each have a different reason for occurring.  Each one is an important opportunity for inspection and adaptation in different areas.  As long as the event purpose is being respected, I have never advocated any specific timing other than the order of Review, Retrospective, Planning.